METHOD FOR SIMULATING PERFORMANCE OF LNG AMBIENT AIR VAPORIZER UNDER FROSTING CONDITION

Abstract

The present disclosure discloses a method for simulating performance of an LNG ambient air vaporizer under a frosting condition, performing site operation test on the LNG ambient air vaporizer to obtain fitting relationship between the outer wall temperature of frosted finned tube and frost layer; transforming a sum of increased frost layer thermal resistance and thermal resistance into equivalent thermal contact resistance, and representing the equivalent thermal contact resistance as a function of the outer wall temperature; establishing a calculation model of the LNG ambient air vaporizer, performing simulation calculation by equivalent heat conduction coefficient, so as to obtain fluid-solid conjugate heat transfer characteristics and vaporization performance of the LNG ambient air vaporizer under the frosting condition. The influences of heat transfer in gas-liquid phase change flow and fluid-solid conjugate heat transfer in the tubes of the vaporizer under the frosting condition are considered in the present disclosure.

Claims

1. A method for simulating performance of an LNG ambient air vaporizer under a frosting condition, includes the following steps: step 1, performing site operation test on the LNG ambient air vaporizer, measuring site ambient temperature T.sub.0, humidity H.sub.0 and atmospheric pressure P.sub.0, and measuring, at a certain operating moment t.sub.n, pressure P.sub.in of liquefied natural gas at a vaporizer inlet and pressure P.sub.out of natural gas at a vaporizer outlet, flow velocity V.sub.in of the liquefied natural gas at the vaporizer inlet and temperature T.sub.in of the liquefied natural gas at the vaporizer inlet, outer wall temperature T.sub.s at different positions of each frosted finned tube of the vaporizer, frost layer temperature T.sub.f, frost layer thickness d.sub.f, and air flow velocity V.sub.a outside a frost layer on a surface of the finned tube, the different positions referring to at least three equidistant point positions of an outer wall of a fin of each frosted finned tube from top to bottom; step 2, performing, by data processing software, fitting analysis on the collected frost layer temperature T.sub.f and frost layer thickness d.sub.f at the different positions of all the frosted finned tubes at the certain operating moment t, and the air flow velocity V.sub.a outside the frost layer on the surface of the finned tube, and data of the outer wall temperature T.sub.s of the finned tube respectively, and obtaining, by a least square method, fitting relational expressions d.sub.f=f.sub.d(T.sub.s)=A.sub.1T.sub.s.sup.2+B.sub.1T.sub.s+C.sub.1, T.sub.f=f (T.sub.s)=A.sub.2T.sub.s+B.sub.2, and V.sub.a=f.sub.V(T.sub.s)=A.sub.3T.sub.s.sup.2+B.sub.3T.sub.s+C.sub.3 between the outer wall temperature T.sub.s of all the frosted finned tubes of the whole vaporizer and the frost layer thickness d.sub.f, the frost layer temperature T.sub.f and the air flow velocity V.sub.a outside the frost layer on the surface of the finned tube at the certain operating moment t.sub.n, A.sub.1, B.sub.1, C.sub.1, A.sub.2, B.sub.2, A.sub.3, B.sub.3, and C.sub.3 in the formula being respectively fitted constants; step 3, establishing a calculation model for an equivalent thermal conductivity coefficient of the LNG ambient air vaporizer during frosting at the certain operating moment t.sub.n: transforming a sum of increased frost layer thermal resistance R.sub.f of each frosted finned tube in unit length at the certain operating moment t.sub.n and thermal resistance R.sub.o of the finned tube body in unit length into equivalent thermal contact resistance R.sub.e of the finned tube in unit length under a non-frosting condition, and representing an equivalent thermal conductivity coefficient .sub.e of the equivalent thermal contact resistance R.sub.e as a function of the outer wall temperature T.sub.s of the frosted finned tube in unit length, the unit length being a length of a minimum mesh during geometric meshing of the vaporizer; wherein ${}_e=F\left(T_s\right)=\frac{1}{\frac{1}{}+\frac{1}{d_{\mathrm{in}}\ln \frac{d_{\mathrm{out}}}{d_{\mathrm{in}}}}.Math.\frac{Z\left(T_s\right)}{\frac{A_2^{}}{A_2}+\frac{A_2^{}}{A_2}.Math.\frac{\mathrm{th}\left(A_m\sqrt{f_V\left(T_s\right)}\right)}{A_m\sqrt{f_V\left(T_s\right)}}}}$ wherein, parameters , d.sub.in, d.sub.out, A.sub.2, A.sub.2, A.sub.2, A.sub.m, and in the formula are all constant values; is a thermal conductivity coefficient of a vaporizer material, namely aluminum alloy; d.sub.in is an internal diameter of the finned tube, and d.sub.out is an external diameter of the finned tube; A.sub.2 is an external surface area of the finned tube in unit length; A.sub.2 is a surface area of a non-fin part outside the finned tube in unit length; A.sub.2 is a surface area of a fin part outside the finned tube in unit length; A.sub.m=/*[36/(*)].sup.1/2, where l is a fin height, and is a thickness of the fin; is a finning coefficient of the finned tube, =A.sub.0/A.sub.2, A.sub.0 is an internal surface area of the finned tube, and A.sub.2 is an external surface area of the finned tube; f.sub.V(T.sub.s) is a function expression relational expression of the air flow velocity V.sub.a outside the frost layer, V.sub.a=f.sub.V(T.sub.s), and T.sub.s is the outer wall temperature at different positions of each frosted finned tube of the vaporizer; and Z(T.sub.s) is a function expression relational expression of the frost layer thermal resistance R.sub.f, R.sub.f=Z(T.sub.s)=d.sub.f/f=f (T.sub.s)/g(T.sub.s), where, g(T.sub.s) is a function expression relational expression of the frost layer thermal conductivity coefficient .sub.f, .sub.f=g(T.sub.s)=0.001202(650e.sup.0.277[(f.sup.T.sup.(T.sup.s.sup.)273.15)]).sup.0.963; step 4, establishing an overall geometric model of the LNG ambient air vaporizer in simulation software, performing meshing and dividing of computational domain, selecting physical models and equations, setting material attributes and boundary conditions of the computational domain, adopting the equivalent thermal conductivity coefficient .sub.e of the finned tube of the vaporizer as the thermal conductivity coefficient during frosting of the vaporizer material, performing solving and initialized setting, and then performing simulation calculation, specifically as follows: S1: establishing, by three-dimensional geometric modeling software, the overall geometric model of the LNG ambient air vaporizer in a ratio of 1:1, and then performing, by finite element meshing software, meshing and dividing of the computational domain of the overall geometric model; dividing the computational domain into an LNG fluid domain, a vaporizer solid domain and an air fluid domain; the LNG fluid domain is a flow region of LNG in an internal channel of the vaporizer; the vaporizer solid domain is a vaporizer body; and the air fluid domain is an air flow region outside the vaporizer body; S2: importing the meshed overall geometric model of the LNG ambient air vaporizer into fluid analysis software, and adopting the LNG fluid domain, the vaporizer solid domain and the air fluid domain as the computational domain; setting a contact surface between the LNG fluid domain and the vaporizer solid domain and a contact surface between the vaporizer solid domain and the air fluid domain as Interface surfaces, and checking a Couple option in Interface setting, so that the corresponding contact surface can complete heat transfer; S3: enabling a gravity model, a multi-phase model, a turbulence model, a boiling phase change model, a continuity equation, a momentum equation, an energy equation and a component transport equation in the fluid analysis software, and adopting a standard wall function method for near-wall processing; adopting a Mixture model as the multi-phase model; adopting a Realizable k-& turbulence model as the turbulence model, and adopting an evaporation-condensation Lee model as the boiling phase change model; S4: setting the material attributes of the computational domain: respectively introducing LNG and NG fluid materials in the fluid analysis software, then setting the LNG fluid material as a first term in the multi-phase model, setting phase change from LNG to NG, and selecting the evaporation-condensation Lee model for a reaction mechanism; introducing an aluminum alloy solid material in the fluid analysis software, adopting physical data in a software material library as parameters of the aluminum alloy solid material, then modifying the thermal conductivity coefficient of the aluminum alloy solid material from the constant value to a value represented by a piecewise polynomial temperature function method, setting the thermal conductivity coefficient of the vaporizer material within a frosting temperature range as the equivalent thermal conductivity coefficient .sub.e=F(T.sub.s), and setting the thermal conductivity coefficient of the vaporizer material within a non-frosting temperature range as the constant value ; introducing a wet air mixed material in the fluid analysis software, the wet air mixed material including air and water vapor, and physical property data in the software material library are adopted as material attributes of the wet air mixed material; S5: setting the boundary conditions of the computational domain: setting an outlet of the LNG fluid domain as a pressure outlet boundary, and adopting the pressure P.sub.out at the vaporizer outlet tested on site as pressure; setting an inlet of the LNG fluid domain as a velocity inlet boundary, and adopting the flow velocity V.sub.in and temperature T.sub.in at the vaporizer inlet tested on site as velocity and temperature; setting a top surface of the air fluid domain above the vaporizer and a side surface of the air fluid domain around the vaporizer as pressure inlet boundaries, adopting the atmospheric pressure P.sub.0 and ambient temperature T.sub.0 tested on site as pressure and temperature, and setting air humidity of the air fluid domain for simulating the air humidity according to the ambient humidity tested on site; setting a bottom surface of the air fluid domain at a bottom of the vaporizer as a pressure outlet boundary; defining the air fluid domain in this step as a hexahedron capable of surrounding the vaporizer, space, except for the vaporizer, inside the hexahedron representing air outside the vaporizer, and the top surface, the side surface and the bottom surface of the air fluid domain referring to a top surface, a side surface and a bottom surface of the hexahedron outside the whole vaporizer; S6: performing initialization setting by adopting a SIMPLE algorithm in the fluid analysis software as a solving method for the geometric model meshes divided in step S1, and then performing calculation simulation on the geometric model established in step S1; stopping calculation and outputting result data from numerical simulation in heat transfer of the vaporizer if a residual variance curve converges and the monitored NG outlet temperature and flow velocity do not change any more; or else, continuing to operate; and step 5, importing the result data from numerical simulation into post-processing software for analysis, displaying a temperature cloud diagram on a surface of the LNG ambient air vaporizer, and a temperature cloud diagram, a velocity cloud diagram and a component cloud diagram of the LNG fluid domain in the tube of the vaporizer in the post-processing software, selecting an outlet section of the LNG fluid domain, to obtain LNG outlet temperature and outlet flow velocity, and selecting the component cloud diagram of the LNG fluid domain, to obtain proportions of a liquid phase section, a two-phase section and a gas-phase section in the fluid domain; and checking temperature, heat flux and other thermal parameters at different position points or sections of the surface of the vaporizer, so as to visually acquire fluid-solid conjugate heat transfer characteristics and vaporization performance of the LNG ambient air vaporizer under the frosting condition.

2. The method for simulating performance of an LNG ambient air vaporizer under a frosting condition according to claim 1, wherein, in the step S6 of the step 4, an LNG volume fraction of an inlet of the LNG fluid domain is set as 1 and temperature is set as 123 K through a Patch function in the fluid analysis software; temperature of the vaporizer solid domain is set as 260 K; a volume fraction of air in the air fluid domain outside the finned tube is set as 1, and temperature is determined through a self-defined formula: T=279+7z, where, T is air temperature, K; and z is a height in an z-axis direction, and z-axis is set to be perpendicular to the bottom of the vaporizer, which is a vertically upward direction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Drawings of the specification constituting a part of the present disclosure are described for further understanding the present disclosure. Schematic embodiments of the present disclosure and descriptions thereof are schematic of the present disclosure, and are not construed as an improper limitation to the present disclosure.

[0025] FIG. 1 is a flowchart of a method for simulating performance of an LNG ambient air vaporizer under a frosting condition according to the present disclosure;

[0026] FIG. 2 is a quarter cross-section diagram of a finned tube of an LNG ambient air vaporizer used for performance simulation under a frosting condition according to the present disclosure;

[0027] FIG. 3 is a principle diagram of transforming a sum of frost layer thermal resistance R.sub.f and thermal resistance R.sub.o of a finned tube body into an equivalent thermal contact resistance R.sub.e of the finned tube adopting a method for simulating performance of an LNG ambient air vaporizer under a frosting condition according to the present disclosure;

[0028] FIG. 4 is a schematic diagram of a relationship between frost layer thermal resistance R.sub.f at different positions of a finned tube and outer wall temperature T.sub.s of the finned tube adopting a method for simulating performance of an LNG ambient air vaporizer under a frosting condition according to the present disclosure;

[0029] FIG. 5 is a diagram of a fitting curve of frost layer thermal resistance and outer wall temperature of a finned tube at different operating times adopting a method for simulating performance of an LNG ambient air vaporizer under a frosting condition according to the present disclosure;

[0030] FIG. 6 is a schematic diagram of a geometric model of a vaporizer adopting a method for simulating performance of an LNG ambient air vaporizer under a frosting condition according to the present disclosure;

[0031] FIG. 7 is a top view of the geometric model of the vaporizer shown in FIG. 6;

[0032] FIG. 8 is a front view of the geometric model of the vaporizer shown in FIG. 6;

[0033] FIG. 9 is a side view of the geometric model of the vaporizer shown in FIG. 6;

[0034] FIG. 10 is a schematic diagram of meshing of a geometric model of a vaporizer adopting a method for simulating performance of an LNG ambient air vaporizer under a frosting condition according to the present disclosure;

[0035] FIG. 11 is a proportion comparison diagram of liquid phase sections, two-phase sections and gas liquid sections in branch tubes of different vaporizer finned tubes in a method for simulating performance of an LNG ambient air vaporizer under a frosting condition according to the present disclosure; and

[0036] FIG. 12 is a comparison diagram of simulation results versus actual measurement results of vaporizer outlet temperature at different operating times in a method for simulating performance of an LNG ambient air vaporizer under a frosting condition according to the present disclosure.

[0037] In the drawings, 1: frost layer outside fin; 2: finned tube of vaporizer; and 3: LNG.

DETAILED DESCRIPTION OF THE PRESENT DISCLOSURE

[0038] It should be noted that the following detailed descriptions are exemplary, which are intended to further explain the present application. Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those ordinarily skilled in the prior art to which the present application pertains.

[0039] It should be noted that the terms used here are not intended to limit the exemplary implementations according to the present application, but are merely descriptive of the specific implementations. Unless otherwise directed by the context, singular forms of terms used here are intended to include plural forms. Besides, it should be also appreciated that, when the terms comprise and/or include are used in the specification, it is indicated that characteristics, steps, operations, devices, assemblies, and/or combinations thereof exist.

[0040] Additionally, any directional indication (such as upper, lower, left, right, front, back, or the like) involved in the embodiments of the present disclosure is only used for explaining relative position relations, movement conditions and the like of components in a certain specific posture (as shown in figures). If the specific posture changes, the directional indications may change accordingly.

[0041] As shown in FIG. 1, a method for simulating performance of an LNG ambient air vaporizer under a frosting condition according to the present disclosure, includes the following steps:

[0042] step 1, site operation test is performed on the LNG ambient air vaporizer, site ambient temperature T.sub.0, humidity H.sub.0 and atmospheric pressure P.sub.0 are measured, and pressure P.sub.in of liquefied natural gas at a vaporizer inlet and pressure P.sub.out of natural gas at a vaporizer outlet, flow velocity V.sub.in of the liquefied natural gas at the vaporizer inlet and temperature T.sub.in of the liquefied natural gas at the vaporizer inlet, outer wall temperature T.sub.s at different positions of each frosted finned tube of the vaporizer, frost layer temperature T.sub.f, frost layer thickness d.sub.f, and air flow velocity V.sub.a outside a frost layer on a surface of the finned tube at a certain operating moment t.sub.n (such as 1 h, 2 h, 4 h or 8 h) are measured, the different positions referring to at least three equidistant point positions of an outer wall of a fin of each frosted finned tube from top to bottom; [0043] step 2, fitting analysis is performed on the collected frost layer temperature T.sub.f and frost layer thickness d.sub.f at the different positions of all the frosted finned tubes at the certain operating moment t.sub.n and the air flow velocity V.sub.a on the outer side of the frost layer on the surface of the finned tube, and data of the outer wall temperature T.sub.s of the finned tube respectively is performed by data processing software (such as SPSS or Origin), and fitting relational expressions d.sub.f=f.sub.d(T.sub.s)=A.sub.1T.sub.s.sup.2+B.sub.1T.sub.s+C.sub.1, T.sub.f=f.sub.T(T.sub.s)=A.sub.2T.sub.s+B.sub.2, and V.sub.a=f.sub.V(T.sub.s)=A.sub.3T.sub.s.sup.2+B.sub.3T.sub.s+C.sub.3 between the outer wall temperature T.sub.s of all the frosted finned tubes of the whole vaporizer and the frost layer thickness d.sub.f, the frost layer temperature T.sub.f and the air flow velocity V.sub.a outside the frost layer on the surface of the finned tube at the certain operating moment t.sub.n are obtained by a least square method, A.sub.1, B.sub.1, C.sub.1, A.sub.2, B.sub.2, A.sub.3, B.sub.3, and C.sub.3 in the formulas being respectively fitted constants.

[0044] In this step, a relationship between the outer wall temperature of each frosted finned tube and the frost layer thickness are integrated to fit a relational expression, that is, data of all the finned tubes at the same operating time t, is collected, the data of all the frosted finned tubes of the vaporizer is fitted, and each relational expression (all the finned tubes are fitted in this relational expression) of d.sub.f=f.sub.d(T.sub.s); T.sub.f=f.sub.T(T.sub.s); V.sub.a=f.sub.V(T.sub.s) is fitted; and different relational expressions may correspond to different operating times. Since temperature distribution of each finned tube is different, a temperature distribution range can be widened by collecting the data of all the finned tubes, which is conducive to improving fitting accuracy. FIG. 2 is a quarter cross-section diagram of a finned tube of an LNG ambient air vaporizer used for performance simulation under a frosting condition according to the present disclosure, which shows the frost layer thickness d.sub.f, an external diameter d.sub.out of the finned tube, an internal diameter d.sub.in of the finned tube, and a thickness of a fin.

[0045] Step 3, a calculation model for an equivalent thermal conductivity coefficient, of the LNG ambient air vaporizer during frosting at the certain operating moment t.sub.n is established: as shown in FIG. 3, a sum of increased frost layer thermal resistance R.sub.f of each frosted finned tube in unit length(the unit length is a length of a minimum mesh during geometric meshing of the vaporizer) at the certain operating moment t.sub.n and thermal resistance R.sub.o of the finned tube body in unit length is transformed into equivalent thermal contact resistance R.sub.e of the finned tube in unit length under a non-frosting condition, and an equivalent thermal conductivity coefficient .sub.e of the equivalent thermal contact resistance R.sub.e is represented as a function of the outer wall temperature T.sub.s of the frosted finned tube in unit length:

[00002]${}_e=F\left(T_s\right)=\frac{1}{\frac{1}{}+\frac{1}{d_{\mathrm{in}}\ln \frac{d_{\mathrm{out}}}{d_{\mathrm{in}}}}.Math.\frac{Z\left(T_s\right)}{\frac{A_2^{}}{A_2}+\frac{A_2^{}}{A_2}.Math.\frac{\mathrm{th}\left(A_m\sqrt{f_V\left(T_s\right)}\right)}{A_m\sqrt{f_V\left(T_s\right)}}}}.$ [0046] wherein, parameters , d.sub.in, d.sub.out, A.sub.2, A.sub.2, A.sub.2, A.sub.m, and in the formula are all constant values; is a thermal conductivity coefficient of a vaporizer material, namely aluminum alloy, W/(m.Math.K), which can be found in professional books (such as Practical Handbook of Nonferrous Metal Materials, Guangdong Science and Technology Press, 2006); d.sub.in is the internal diameter of the finned tube, and d.sub.out is the external diameter of the finned tube, m, which can be obtained through a design drawing; A.sub.2 is an external surface area of the finned tube in unit length, m.sup.2; A.sub.2 is a surface area of a non-fin part outside the finned tube in unit length, m.sup.2; A.sub.2 is a surface area of a fin part outside the finned tube in unit length, m.sup.2; A.sub.2, A.sub.2, and A.sub.2 can be calculated from the design drawing; A.sub.m=!*[36/(*)].sup.1/2, where l is a fin height, is the thickness of the fin, and m, l, and can be obtained from the design drawing; is a finning coefficient of the finned tube, A.sub.0/A.sub.2, A.sub.0 is an internal surface area of the finned tube, and A.sub.2 is an external surface area of the finned tube, m.sup.2, which can be calculated from the design drawing. f.sub.V(T.sub.s) is a function expression relational expression of the air flow velocity V.sub.a outside the frost layer, and T.sub.s is the outer wall temperature K at different positions of each frosted finned tube of the vaporizer; and Z(T.sub.s) is a function expression relational expression of the frost layer thermal resistance R.sub.f, R.sub.f=Z(T.sub.s)=d.sub.f/.sub.f=f.sub.d(T.sub.s)/g(T.sub.s), where, g(T.sub.s) is a function expression relational expression of the frost layer thermal conductivity coefficient .sub.f, .sub.f=g(T.sub.s)=0.001202(650e.sup.0.277[(f.sup.T.sup.(T.sup.s.sup.)273.15)]).sup.0.963.

[0047] Since the outer wall temperature T.sub.s and the frost layer thickness d.sub.f on each section of the finned tube are different, the frost layer thermal resistance R.sub.e of the finned tube in unit length is also different (as shown in FIG. 4), the relational expression is established between the frost layer thermal resistance R.sub.f and T.sub.s, and the frost layer thermal resistance R.sub.f in each section of the finned tube in unit length may be represented by the outer wall temperature T.sub.s of the finned tube; therefore, the increased frost layer thermal resistance R.sub.e of the whole finned tube is not a constant value for each section any more, but is a linear value changing with the outer wall temperature T.sub.s of each section of the finned tube; finally, the thermal conductivity coefficient .sub.e of the equivalent thermal contact resistance R.sub.e of the frosted finned tube is represented as the function of the outer wall temperature T.sub.s of the finned tube; and the increased frost layer thermal resistance R.sub.f in each section of each finned tube in unit length and the thermal conductivity coefficient we are different, however, the thermal conductivity coefficients of all the sections are calculated through the function relational expression .sub.e=F(T.sub.s).

[0048] A deduction process of the function relational expression. .sub.e=F(T.sub.s) of the thermal conductivity coefficient .sub.e of the equivalent thermal contact resistance R.sub.e of the frosted finned tube and the outer wall temperature T.sub.s of the frosted finned tube is as follows:

[0049] Firstly, let a total heat transfer coefficient K.sub.f (the total heat transfer coefficient is obtained according to a calculation formula for a heat transfer process through a ribbed wall in Heat Transfer 5th Edition (Higher Education Press, 2019) be equivalent to a total heat transfer coefficient K of the finned tube under a non-frosting condition, and then an expression of the equivalent thermal conductivity coefficient .sub.e of the vaporizer material, namely aluminum alloy is obtained through transposition, as follows:

[00003]$\frac{1}{\frac{1}{h_{\mathrm{in}}}+\frac{d_{\mathrm{in}}}{}\ln \frac{d_{\mathrm{out}}}{d_{\mathrm{in}}}+\frac{1}{.Math.}\left(\frac{1}{h_{\mathrm{out}}}+R_f\right)}=K_f=K=\frac{1}{\frac{1}{h_{\mathrm{in}}}+\frac{d_{\mathrm{in}}}{{}_e}\ln \frac{d_{\mathrm{out}}}{d_{\mathrm{in}}}+\frac{1}{.Math..Math.h_{\mathrm{out}}}}$${}_e=\frac{1}{\frac{1}{}+\frac{1}{d_{\mathrm{in}}\ln \frac{d_{\mathrm{out}}}{d_{\mathrm{in}}}}.Math.\frac{R_f}{.Math.}}$

[0050] In the formula, K.sub.f is the total heat transfer coefficient of the finned tube of the LNG ambient air vaporizer under the frosting condition, W/(m.sup.2.Math.K); K is the total heat transfer coefficient of the finned tube of the LNG ambient air vaporizer under the non-frosting condition, W/(m.sup.2.Math.K); h.sub.in is a surface heat transfer coefficient in the finned tube, W/(m.sup.2.Math.K); d.sub.in is the internal diameter of the finned tube, m; is the thermal conductivity coefficient of the vaporizer material, namely aluminum alloy, W/(m.Math. K); d.sub.out is the external diameter of the finned tube, m; is fin efficiency (the fin efficiency=actual heat dissipating capacity of the surface of the fin/heat dissipating capacity assuming that the outer wall temperature of the fin is equal to fin root temperature); is a finning coefficient of the finned tube; h.sub.out is a heat transfer coefficient on an air side outside the finned tube, W/(m.sup.2.Math.K); R.sub.f is the frost layer thermal resistance, (m.sup.2.Math.K)/W; and .sub.e is the equivalent thermal conductivity coefficient of the vaporizer material, namely the aluminum alloy, W/(m.Math.K).

[0051] Parameter can be obtained in professional books (such as Practical Handbook of Nonferrous Metal Materials, Guangdong Science and Technology Press, 2006); d.sub.in and d.sub.out can be obtained from the design drawing; , , h.sub.out, and R.sub.f can be calculated through the following formula; and h.sub.in is eliminated in calculation, which does not need to be solved.

[0052] The heat transfer coefficient h.sub.out on the air side outside the finned tube is represented as an additive value of a convection heat exchange coefficient h.sub.out,d on the air side outside the tube and a radiation heat exchange coefficient h.sub.out,r on the air side outside the tube: h.sub.out=h.sub.out,d+h.sub.out,r; and since the outer wall temperature T.sub.s of the finned tube has little influence on the convection heat exchange coefficient h.sub.out,d on the air side, the heat transfer coefficient can be calculated through the air flow velocity V.sub.a on the outer side of the frost layer on the surface of the finned tube measured on site: h.sub.out=18V.sub.a.

[0053] A calculation formula of the fin efficiency is:

[00004]$=\frac{A_2^{}+A_2^{}{}_f}{A_2}$

[0054] In the formula, A.sub.2 is the surface area outside the finned tube, m.sup.2; A.sub.2 is a surface area of a non-fin part outside the finned tube in unit length, m.sup.2; A.sub.2 is a surface area of a fin part outside the finned tube in unit length, m.sup.2; A.sub.2, A.sub.2, and A.sub.2 can be calculated from the design drawing; and .sub.f is fin surface efficiency, and a calculation formula is:

[00005]${}_f=\frac{\mathrm{th}\left(ml\right)}{ml}$$m=\sqrt{\frac{2h_{\mathrm{out}}}{}}$

[0055] In the formula, m is the fin coefficient; l is the fin height, m; is the thermal conductivity coefficient of the vaporizer material, namely the aluminum alloy, W/(m.Math.K); is the fin thickness, m; both l and can be obtained from the design drawing; h.sub.out is the heat transfer coefficient on the air side outside the finned tube, W/(m.sup.2.Math.K), when radiation heat exchange on the air side is neglected, h.sub.out can be calculated according to the air flow velocity V.sub.a outside the frost layer h.sub.out=18V.sub.a, and h.sub.out is expressed as the function h.sub.out=18V.sub.a=18f.sub.V(T.sub.s) with the outer wall temperature T.sub.s of the finned tube according to the fitting relationship V.sub.a=f.sub.V(T.sub.s) between the outer wall temperature T.sub.s of the finned tube and the air flow velocity outside the frost layer, m=[2*18*f.sub.V(T.sub.s)/(*)].sup.1/2. Let A.sub.m=l*[36/(*)].sup.1/2, then .sub.f=th(A.sub.m*f.sub.V(T.sub.s).sup.1/2)/(A.sub.m*f.sub.V(T.sub.s).sup.1/2).

[0056] A calculation formula of the finning coefficient 8 of the finned tube is:

[00006]$=\frac{A_0}{A_2}$

[0057] In the formula, A.sub.0 is the internal surface area of the finned tube, m.sup.2, and A.sub.2 is the external surface area of the finned tube, m.sup.2, which can be calculated from the design drawing.

[0058] A calculation formula of the frost layer thermal resistance R.sub.f is:

[00007]$R_f=\frac{d_f}{{}_f}$

[0059] In the formula, d.sub.f is the frost layer thickness, mm; .sub.f is the frost layer thermal conductivity coefficient, W/(m.Math. K), which mainly depends on frost layer density .sub.f and can be calculated through Sanders relational expression (reference: Seker D, Karatas H, Egrican N .Frost formation on fin-and-tube heat exchangers. Part I-Modeling of frost formation on fin-and-tube heat exchangers [J]. International Journal of Refrigeration, 2004, 27 (4): 367-374:

.sub.f=0.001202.sub.f.sup.0.963

[0060] In the formula, .sub.f is the frost layer density, kg/m.sup.3; and a calculation formula is as follows:

[00008]${}_f=650e^{0.277\left(T_f-273.15\right)}$

[0061] In the formula, T.sub.f is the frost layer temperature, K.

[0062] The above two formulas are combined to obtain a formula of the frost layer thermal conductivity coefficient .sub.f changing with the frost layer temperature T.sub.f, as follows:

[0063] The function expression of the frost layer thermal conductivity coefficient is

[00009]${}_f=0.001202{\left(650e^{0.277\left(T_f-273.15\right)}\right)}^{0.963}$

and the outer wall temperature T.sub.s of the finned tube is obtained according to the fitting relational expression T.sub.f=f.sub.T(T.sub.s) between the outer wall temperature T.sub.s of the finned tube and the frost layer temperature T.sub.f, as follows:

[00010]${}_f=g\left(T_s\right)=0.001202{\left(650e^{0.277\left(f_T\left(T_s\right)-273.15\right)}\right)}^{0.963}$

[0064] The frost layer thermal resistance R.sub.f is expressed as the function R.sub.f=d.sub.f/.sub.f=.sub.d(T.sub.s=f (T.sub.s)/g(T.sub.s)=Z(T.sub.s) of the outer wall temperature T.sub.s of the finned tube. FIG. 5 is a diagram of a fitting curve of frost layer thermal resistance R.sub.f and the outer wall temperature T.sub.s of the finned tube at different operating times.

[0065] R.sub.f=Z(T.sub.s), =(A.sub.2+A.sub.2n.sub.f)/A.sub.2, and n.sub.f=th(A.sub.mf.sub.V(T.sub.s).sup.1/2)/(A.sub.mf.sub.V(T.sub.s).sup.1/2) are substituted into the expression of the equivalent thermal conductivity coefficient .sub.e of the vaporizer material, namely the aluminum alloy, to obtain:

[00011]${}_e=\frac{1}{\frac{1}{}+\frac{1}{d_{\mathrm{in}}\ln \frac{d_{\mathrm{out}}}{d_{\mathrm{in}}}}.Math.\frac{R_f}{.Math.}}=\frac{1}{\frac{1}{}+\frac{1}{d_{\mathrm{in}}\ln \frac{d_{\mathrm{out}}}{d_{\mathrm{in}}}}.Math.\frac{Z\left(T_s\right)}{\frac{A_2^{}+A_2^{}{n}_f}{A_2}.Math.}}=\frac{1}{\frac{1}{}+\frac{1}{d_{\mathrm{in}}\ln \frac{d_{\mathrm{out}}}{d_{\mathrm{in}}}}.Math.\frac{Z\left(T_s\right)}{\frac{A_2^{}}{A_2}+\frac{A_2^{}}{A_2}.Math.\frac{\mathrm{th}\left(A_m\sqrt{f_V\left(T_s\right)}\right)}{A_m\sqrt{f_V\left(T_s\right)}}}}$

[0066] In the formula, , d.sub.in, d.sub.out, A.sub.2, A.sub.2, A.sub.2, A.sub.m, and are all constant values; and the equivalent thermal conductivity coefficient .sub.e of the finned tube of the vaporizer is expressed as the function .sub.e=F(T.sub.s) of the outer wall temperature T.sub.s of the finned tube.

[0067] Step 4, an overall geometric model of the LNG ambient air vaporizer is established in simulation software, meshing and dividing of computational domain are performed, physical models and equations are selected, material attributes and boundary conditions of the computational domain are set, the equivalent thermal conductivity coefficient de of the finned tube of the vaporizer is adopted as the thermal conductivity coefficient during frosting of the vaporizer material, solving and initialization setting are performed, and then simulation calculation is performed. Details are as follows:

[0068] S1: as shown in FIG. 6 to FIG. 9, the overall geometric model of the LNG ambient air vaporizer in a ratio of 1:1 is established through three-dimensional geometric modeling software (such as SolidWorks or ANSYS DesignModeler), then the overall geometric model is subjected to meshing (as shown in FIG. 10, which is a schematic diagram of geometric model meshing through ANSYS Meshing) and dividing of the computational domain through finite element meshing software (such as ICEM CFD or ANSYS Meshing). The meshing needs to take the number, density and quality of meshes into account, so as to improve calculation efficiency and accuracy; the computational domain are divided into an LNG fluid domain, a vaporizer solid domain and an air fluid domain; the LNG fluid domain is a flow region of LNG in an internal channel of the vaporizer; the vaporizer solid domain is a vaporizer body; and the air fluid domain is an air flow region outside the vaporizer body.

[0069] S2: the meshed overall geometric model of the LNG ambient air vaporizer is imported into fluid analysis software (such as ANSYS Fluent), and the LNG fluid domain, the vaporizer solid domain and the air fluid domain are adopted as the computational domain; and a contact surface between the LNG fluid domain and the vaporizer solid domain and a contact surface between the vaporizer solid domain and the air fluid domain are set as Interface surfaces, and a Couple option checked set in Interface setting, so that the corresponding contact surface can complete heat transfer.

[0070] S3: a gravity model, a multi-phase model, a turbulence model, a boiling phase change model, a continuity equation, a momentum equation, an energy equation and a component transport equation are enabled in the fluid analysis software, and a standard wall function method is adopted for near-wall processing; a Mixture model is adopted as the multi-phase model; and a Realizable k-epsilon turbulence model is adopted as the turbulence model, and an evaporation-condensation Lee model is adopted as the boiling phase change model.

[0071] S4: the material attributes of the computational domain are set:

[0072] LNG and NG fluid materials are respectively introduced in the fluid analysis software, physical parameter data about LNG and NG in relevant books (Technical Handbook of Liquefied Natural Gas, China Machine Press, 2010) are adopted as material parameters, then the LNG fluid material is set as a first term in the multi-phase model, phase change from LNG to NG is set, and the evaporation-condensation Lee model is selected for a reaction mechanism; an aluminum alloy solid material is introduced in the fluid analysis software, physical data in a software material library is adopted as parameters of the aluminum alloy solid material, then the thermal conductivity coefficient of the aluminum alloy solid material is modified from the constant value to a value represented by a piecewise polynomial temperature function method, the thermal conductivity coefficient of the vaporizer material within a frosting temperature range is set as the equivalent thermal conductivity coefficient .sub.e=F(T.sub.s), and the thermal conductivity coefficient of the vaporizer material with a non-frosting temperature range is set as the constant value ; and a wet air mixed material is introduced in the fluid analysis software, the wet air mixed material including air and water vapor, and physical property data in the software material library are adopted as material attributes of the wet air mixed material.

[0073] S5: the boundary conditions of the computational domain are set:

[0074] An outlet of the LNG fluid domain is set as a pressure outlet boundary, and the pressure P.sub.out at the vaporizer outlet tested on site is adopted as pressure; an inlet of the LNG fluid domain is set as a velocity inlet boundary, and the flow velocity V.sub.in and temperature T.sub.in at the vaporizer inlet tested on site are adopted as velocity and temperature; a top surface of the air fluid domain above the vaporizer and a side surface of the air fluid domain around the vaporizer are set as pressure inlet boundaries, the atmospheric pressure P.sub.0 and ambient temperature T.sub.0 tested on site are set as pressure and temperature, and air humidity of the air fluid domain for simulating the air humidity is set according to the ambient humidity tested on site; and a bottom surface of the air fluid domain at a bottom of the vaporizer is set as a pressure outlet boundary.

[0075] In this step, the air fluid domain is defined as a hexahedron capable of surrounding the vaporizer, space, except for the vaporizer, inside the hexahedron represents air outside the vaporizer, and the top surface, the side surface and the bottom surface of the air fluid domain refer to a top surface, a side surface and a bottom surface of the hexahedron outside the whole vaporizer.

[0076] S6: initialization setting is performed by adopting a SIMPLE algorithm in the fluid analysis software as a solving method for the geometric model meshes divided in step S1, and then calculation simulation is performed on the geometric model established in step S1; calculation stops and result data from numerical simulation in heat transfer of the vaporizer is output if a residual variance curve converges and the monitored NG outlet temperature and flow velocity do not change any more; otherwise, it proceeds to operate.

[0077] Furthermore, in order to make the initialization result as close as possible to the actual physical result to ensure the stability of the calculation process and increase the convergence velocity, an LNG volume fraction of an inlet of the LNG fluid domain is set as 1 and temperature is set as 123 K through a Patch function in the fluid analysis software; temperature of the vaporizer solid domain is set as 260 K; a volume fraction of air in the air fluid domain outside the finned tube is set as 1, and temperature is determined through a self-defined formula: T=279+7z, where, T is air temperature, K; and z is a height in an z-axis direction (z-axis is set to be perpendicular to the bottom of the vaporizer, which is a vertically upward direction), m.

[0078] Step 5, the result data from numerical simulation is imported into post-processing software (such as Tecplot or Ensight) for analysis, a temperature cloud diagram on a surface of the LNG ambient air vaporizer, and a temperature cloud diagram, a velocity cloud diagram and a component cloud diagram of the LNG fluid domain in the tube of the vaporizer are displayed in the post-processing software, an outlet section of the LNG fluid domain is selected, to obtain LNG outlet temperature and outlet flow velocity, and the component cloud diagram of the LNG fluid domain is selected, to obtain proportions of a liquid phase section, a two-phase section and a gas-phase section in the fluid domain; and temperature, heat flux and other thermal parameters at different position points or sections of the surface of the vaporizer are checked, so as to visually acquire fluid-solid conjugate heat transfer characteristics and vaporization performance of the LNG ambient air vaporizer under the frosting condition.

[0079] Analysis results are shown in FIG. 11 and FIG. 12, and FIG. 11 shows proportions of liquid phase sections, two-phase sections and gas liquid sections in branch tubes of different vaporizer finned tubes under a frosting condition; and FIG. 12 shows comparison of simulation results versus actual measurement results of vaporizer outlet temperature at different operating times, and it can be shown that errors between the simulation results of the method and the actual measurement results are small, thereby realizing good accuracy.

[0080] The above description is only the preferred embodiments of the present application, and is not intended to limit the present application, and for those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall fall within the protection scope of the present application.