Advanced control two phase heat transfer loop

Abstract

The advanced control heat transfer loop apparatus (1) for heat transfer and thermal control applications uses a two-phase fluid as a working media and comprises at least one evaporator (2) to be connected with a heat source and comprising primary capillary pump (4), a thermal stabilization-compensation chamber (3) being attached to the at least one evaporator (2), at least one condenser (24) to be connected with a heat sink, liquid lines (22) and vapor lines (23) connecting the at least one evaporator (2) and the at least one condenser (24), a remote compensation chamber (20), temperature sensors (27) for detecting the temperature of the remote compensation chamber (20) and at the thermal stabilization compensation chamber (3) attached to the at least one evaporator (2), at least one heating element (19) for heating the remote compensation chamber (20), and a controller (28). The controller (28) is configured to monitor the temperatures detected by the sensors (27) and to control the heating element (19) in such a way that the value of the difference ΔT.sub.Control between the temperature of the remote compensation chamber (20) and the temperature of the thermal stabilization-compensation chamber (3) attached to the at least one evaporator (2) is positive.

Claims

1. Advanced control heat transfer loop apparatus for heat transfer and thermal control applications, using a two-phase fluid as a working media and comprising: at least one evaporator to be connected with a heat source and comprising a primary capillary pump, a thermal stabilization-compensation chamber being attached to said at least one evaporator, at least one condenser to be connected with a heat sink, liquid lines and vapor lines connecting said at least one evaporator and said at least one condenser, a remote compensation chamber, temperature sensors for detecting the temperature of said remote compensation chamber and at said thermal stabilization compensation chamber attached to said at least one evaporator, at least one heating element for heating said remote compensation chamber, and a controller, wherein said primary capillary pump is connected to said thermal stabilization compensation chamber by means of a secondary capillary pump providing a gravity field independent operation of said evaporator, wherein said controller is configured to monitor the temperatures detected by said sensors and to control said heating element in such a way that the value of the difference ΔT.sub.Control between the temperature of said remote compensation chamber and the temperature of said thermal stabilization compensation chamber attached to said at least one evaporator is positive.

2. Advanced control heat transfer loop for heat transfer and thermal control applications according to claim 1, wherein the positive value of the difference ΔT.sub.Control between the temperature of said remote compensation chamber and the temperature of said thermal stabilization compensation chamber attached to said at least one evaporator is a fixed value.

3. Advanced control heat transfer loop for heat transfer and thermal control applications according to claim 1, wherein the positive value of the difference ΔT.sub.Control between the temperature of said remote compensation chamber and the temperature of said thermal stabilization-compensation chamber attached to said at least one evaporator is a value variable according to a function of modes of operation of the advanced control heat transfer loop.

4. Advanced control heat transfer loop for heat transfer and thermal control applications according to claim 1, wherein the controller is configured to provide a stabilization of the temperature of the heat source at a fixed value above the difference ΔT.sub.Control between the temperature of said remote compensation chamber and the temperature of said thermal stabilization-compensation chamber attached to said at least one evaporator.

5. Advanced control heat transfer loop for heat transfer and thermal control applications, according to claim 1, wherein said primary capillary pump comprises outer vapor channels to collect and remove heat from a cooled equipment and inner vapor channels to collect and remove vapor bubbles produced by parasitic heat leak penetrating through said primary capillary pump.

6. Advanced control heat transfer loop for heat transfer and thermal control applications according to claim 1, wherein said remote compensation chamber comprises an internal capillary structure to assure continuous presence of liquid phase in said inlet of the liquid feeding line to said remote compensation chamber.

7. Advanced control heat transfer loop for heat transfer and thermal control applications according to claim 1, further comprising a liquid pump in the liquid line.

8. A method for operating an advanced control heat transfer loop apparatus for heat transfer and thermal control applications, the apparatus using a two-phase fluid as a working media and comprising: at least one evaporator to be connected with a heat source and comprising a primary capillary pump, a thermal stabilization-compensation chamber being attached to said at least one evaporator, at least one condenser to be connected with a heat sink, liquid lines and vapor lines connecting said at least one evaporator and said at least one condenser, a remote compensation chamber, at least one heating element for heating said remote compensation chamber, and a controller, wherein said primary capillary pump is connected to said thermal stabilization compensation chamber by means of a secondary capillary pump providing a gravity field independent operation of said evaporator, wherein the temperatures of said remote compensation chamber and at said thermal stabilization compensation chamber attached to the at least one evaporator are detected and monitored and the heating element is controlled in such a way that the value of the difference ΔT.sub.Control between the temperature of said remote compensation chamber and the temperature of said thermal stabilization-compensation chamber attached to said at least one evaporator is positive.

9. The method according to claim 8, wherein the positive value of the difference ΔT.sub.Control between the temperature of said remote compensation chamber and the temperature of said thermal stabilization compensation chamber attached to said at least one evaporator is a fixed value.

10. The method according to claim 8, wherein the positive value of the difference ΔT.sub.Control between the temperature of said remote compensation chamber and the temperature of said thermal stabilization-compensation chamber attached to said at least one evaporator is a value variable according to a function of modes of operation of said advanced control heat transfer loop.

11. The method according to claim 8, wherein a stabilization of the temperature of the heat source at a fixed value above the difference ΔT.sub.Control between the temperature of said remote compensation chamber and the temperature of said thermal stabilization-compensation chamber attached to said at least one evaporator is provided.

Description

DESCRIPTION OF THE DRAWINGS

(1) The features, objects and advantages of the invention will become apparent by reading this description in conjunction with the accompanying drawings, in which:

(2) FIGS. 1a and 1b show schematic views of the ACHTL device of the invention having a remote compensation chamber and two evaporators.

(3) FIG. 2 shows a cross section of the ACHTL evaporator.

(4) FIGS. 3a and 4a show a pressure-temperature diagram of an ACHTL thermodynamic cycle which illustrates the main principle of ACHTL operation.

(5) FIGS. 3b and 4b show ACHTL schematics which correspond to pressure-temperature diagrams shown on FIGS. 3a and 4a.

DETAILED DESCRIPTION OF THE INVENTION

(6) The present invention is illustrated by FIGS. 1a, 1b, 2. When a heat input flow 11 is supplied to an evaporator 2 through an evaporator saddle 9 by a heat releasing equipment or a heat source, the heat evaporates working liquid. The saddle 9 is made from highly thermally conductive material (for instance, aluminium or copper) and it is needed to connect (mechanically and thermally) the evaporator 2 which typically has a cylindrical shape with the heat source (typically, a flat surface, for instance, an electronic chip). The vapor flows from the evaporator 2 to a condenser 27 through a vapor transport line 23, where it is condensed. After that, the working liquid returns to a stabilization-compensation chamber 3 and to the evaporator 2 through a liquid transport line 24, to be again evaporated on the external surface of a primary capillary pump 4 installed in the evaporator 2.

(7) Unlike ordinary LHP systems, the proposed ACHTL device 1 of the invention is controlled by a remote compensation chamber 20, in which two-phases always co-exist. Unlike ordinary CPL systems the stabilization-compensation chamber 3 is provided in the ACHTL for each evaporator 2. The stabilization-compensation chamber 3 is connected and attached to the evaporator 2 and together with an advanced control scheme serves for reliable supplying of the primary capillary pump 4 with a sufficient amount of subcooled liquid in any operational conditions, even in very unfavorable transient ones.

(8) The ACHTL device 1 comprises at least one evaporator 2 (in FIGS. 1a and 1b two evaporators) comprising the primary capillary pump 4, at least one thermal stabilization-compensation chamber 3, at least one condenser 24, liquid and vapor lines 22 and 23, a single remote compensation chamber 20 comprising a capillary structure 21, temperature sensors 27 installed at all compensation chambers 3 and 22 of the system, at least one heating element 19 installed at the remote compensation chamber, and an automatic controller 28. The remote compensation chamber 20 is hydraulically connected with the thermal stabilization-compensation chamber 3 and the condenser 24 through a liquid feeding line 18 and liquid line 22. The primary capillary pump 4 of the evaporator 2 serves to absorb heat from the heat source 11 (equipment, which has to be thermally controlled), and to provide fluid/heat continuous circulation between the evaporator 2 connected with heat source 11 and the condenser 24, which is attached to a heat sink 25. The main part of the absorbed heat is used for evaporation of working fluid. Released vapor flows through vapor removing channels 6 and then through vapor line 23 towards the condenser 24 where heat stored in the vapor phase is released to the heat sink 25 by condensation. The vapor flow 13 is caused by temperature and corresponding pressure gradients between evaporator 2 and condenser 24. A small part of total heat input 11 can reach the central core of the wick of the primary capillary pump 4. This is a parasitic heat leak 12 because this heat degrades HTL conductance performance and has to be minimized. A secondary capillary pump 5 is located inside the wick of the primary capillary pump 4 and inside of the thermal stabilization-compensation chamber 3 and serves to distribute and supply the wick of the primary capillary pump 4 with liquid to provide fluid/heat intermittent circulation in transient regimes of operation of the ACHTL, including removing of internal parasitic heat leak 12 through vapor bubbles removing channels 8 of the primary capillary pump 4 by convection and condensation of the bubbles 10 generated on the inner wall of the pump 4.

(9) The tolerance of the ACHTL system 1 to the vapor bubble 10 appearance in the central core of wick of the primary capillary pump 4 in transient regimes and absence of those bubbles in steady state regimes of operation of the ACHTL is secured by the presence of stabilization compensation chamber(s) 3 together with an advanced method of temperature control. Parasitic heat leak is minimal if boiling (and the bubble flow 10, corresponding to this process) does not take place inside of primary capillary pump 4. In this situation ACHTL has maximum performance. It means that only liquid phase 15 is presented in the central core of primary capillary pump 4, in secondary capillary pump 5 and in the thermal stabilization-compensation chamber 3 in steady state mode of ACHTL device 1 operation. However in transient regimes when the heat input 11 power or/and temperature of the heat sink 25 are changing rapidly it is often impossible to avoid the generation of bubbles 10. Then, the bubbles 10 move to the stabilization compensation chamber 3 where they are condensing. This is possible only if this chamber has sufficiently low temperature during all transient modes of ACHTL operation. The advanced control scheme and method guarantee proper operation of the system in all regimes. The control scheme consists of a controller 28, temperature sensors 27 and a heater 19 at the remote compensation chamber 20. The controller 28 controls the heating of the remote compensation chamber 20 in such a way that the temperature of the remote compensation chamber T.sub.RCC is always above the temperature of any of thermal stabilization-compensation chambers T.sub.SCC according to the control algorithm 30.

(10) The ACHTL device 1 of the invention can be of the type of a single evaporator-condenser or of multiple evaporators (and/or condensers) embodiments. To enhance ACHTL performance a pump 31 can be installed in the liquid line 22, as shown in FIG. 1b. The ACHTL device 1 of the invention comprises the following components: at least one evaporator 2; one remote compensation chamber 20 in a two-phase condition for temperature control functions and for managing changes of liquid phase volume. Presence of one remote compensation chamber 20 provides expandability in embodiments having multiple evaporators 2; in that case there is no need for the stabilization-compensation chambers 3 having a large volume, as they can have a minimal volume, enough to manage and ensure tolerance of vapor bubbles 10 in transient regimes; at least one condenser 24; a vapor line 23 and a liquid line 22; an advanced control scheme comprising temperature sensors 27 installed at every compensation chamber 3, 20, a controller 28 and a heating element 19 for the remote compensation chamber 20.

(11) The numerals shown in FIGS. 1a-1b, 2 and 3a-3b represent the following: 1—Advanced Control Heat Transfer Loop device; 2—Evaporator; 3—Thermal Stabilization compensation chamber; 4—Primary capillary pump; 5—Secondary capillary pump; 6—Vapor removing channels outside the wick of the primary capillary pump; 7—Bayonet tube; 8—Vapor bubbles removing channels inside of the wick of the primary capillary pump; 9—Evaporator saddle; 10—Vapor bubbles in central core of wick of the primary capillary pump; 11—Heat input flow; 12—Heat leak flow into central core of wick of the primary capillary pump; 13—Vapor flow direction; 14—Liquid flow direction; 15—Liquid; 16—Vapor; 17—Vapor-liquid front in remote compensation chamber; 18—Liquid feeding line to/from remote compensation chamber; 19—Heater at remote compensation chamber; 20—Remote compensation chamber; 21—Capillary structure inside of remote compensation chamber; 22—Liquid line; 23—Vapor line; 24—Condenser saddle or plate; 25—Heat sink; 26—Heat output flow; 27—Temperature sensor; 28—Analog or digital controller; 29—Electrical conductor; 30—Control algorithm 31—Pump

(12) The explanation of the physical model of the advanced control is illustrated on FIGS. 3a, 3b and 4a, 4b. Since the device of the invention is an evaporating-condensing heat transfer apparatus, it operates around the vapor-liquid saturation line SL. Two closed thermodynamic cycles of ACHTL operation are shown on the pressure (P)—temperature (T) diagrams in FIGS. 3a and 4a. The points from 100 to 112 on the diagrams correspond to certain thermodynamic states of working fluid in different locations of ACHTL as shown in FIGS. 3b and 4b. At position (100) liquid evaporates from the external surface of the wick of the primary capillary pump 4 and flows to the outlet of the evaporator 2, path (100-101). In this step, some vapor overheating can take place. After that, vapor flows into the vapor line 23 (path 101-102), the temperature of vapor in the vapor line 3 being maintained close to constant (there is no heat exchange with the ambient) though the vapor pressure in the line 23 is reduced. In the condenser 24 (path 102-103-104-105), vapor is cooled up to saturation state (102-103), then condensed (103-104) and the liquid condensate is further subcooled (104-105). Pressure is further reduced on the way of liquid to evaporator 2 (105-106-107) due to friction losses in the conduit 22. Flowing down in the line (105-106-107) the liquid can keep the constant temperature, and can be cooled or be heated (as it is shown on the diagram) depending on the thermal environment conditions of the liquid line 22. In the remote compensation chamber 20, the vapor 16 and liquid 15 phases are always presented in equilibrium and the temperature of this chamber is the defining point for entire HTL since whole cycle depends on this point (110). The flow in the line 18 can be presented only in transient regimes, therefore there is no pressure drop between the points (106) and (110). The subcooled liquid from the condenser 24 is first heated in the stabilization compensation chamber 3 (107-108) and afterwards inside of the central core of the wick of the primary capillary pump 4 (108-109) absorbing the parasitic heat leak 12. The liquid passes the saturation line inside the wick (111) but it can not boil due to constrained conditions inside of the wick micro capillaries (surface tension forces are preventing growing of bubbles). From point (111) to (112) liquid is superheated and pressure is further reduced during the filtration through the porous structure (109-111-112). The cycle is closed at vapor-liquid interface-meniscus where evaporation takes place (112-100). Point (112) corresponds to the liquid phase just under the meniscus, point (100) corresponding to the vapor phase just above the meniscus.

(13) As it is clear from the diagram in FIG. 3a, an insufficient subcooling will lead to the reduction of the temperature difference between points (107) and (112) and finally to a situation when the points (109) and (111) become equal. In this case the liquid will start to boil inside of the central core of wick of the primary capillary pump 4, which will lead to a sudden increase of parasitic heat leak, to degradation of HTL thermal conductance and finally to the dryout of the wick and to interruption of the fluid circulation (failure of HTL operation). Thus, the liquid subcooling (104-105) is the fundamental parameter for proper and stable operation of any HTL.

(14) Especially important for transient regimes are rapid large changes of heat sink, heat source or ambient conditions such as heat source input power, condenser and ambient temperatures or heat exchange conditions that can provoke dryout of the evaporator(s) due to insufficient subcooling.

(15) To guarantee proper operation of the HTL in all regimes it is proposed to control the temperature of remote compensation chamber 20 in such a way that for all scenarios of ACHTL operation there is enough liquid subcooling to compensate parasitic heat leak before point (109) will converge with point (111): dryout. As it is shown on FIG. 4a the increase of temperature difference between remote and stabilization compensation chambers T.sub.110-T.sub.108=ΔT.sub.RCC-SCC will cause the increase of the overall subcooling temperature drop T.sub.104-T.sub.106=T.sub.Subcool. The necessary differences of temperatures can be obtained by heating of remote compensation chamber (FIG. 4a, heat input 11 to remote compensation chamber 20). Due to this heating the liquid from the remote compensation chamber 20 is pushed into condenser 24 (vapor is expanding). It leads to a larger length of the liquid path in the condenser and finally to an increase of subcooling rate of the liquid.

(16) The following transient regimes of ACHTL can be identified: 1. Startup. This event is most stressful and less predictable for the system since it depends not only on initial temperatures of ACHTL elements and applied power to evaporator but also on the original allocation of vapor and liquid phases inside ACHTL. 2. Shut down. On case of multiple evaporators the effect of ACHTL power switching off for one or several evaporators, keeping rest of the evaporators operating can lead to sudden vapor and liquid flows redirections and to strong oscillations end even to dryout of the system. 3. Evaporator input power increase 4. Evaporator input power decrease 5. Condenser temperature increase 6. Condenser temperature decrease 7. Combinations of conditions 3-5, 3-6, 4-5, 4-6 for one-evaporator one-condenser ACHTL 8. Multiple combinations of conditions 1-2-3-4-5-6 for multi-evaporator multi-condenser ACHTL 9. Change of transport lines environmental thermal conditions which also can be combined with all above mentioned cases 1-8.

(17) The more complex the system is, the more combinations are possible, the more it is difficult to predict and test the system behavior in transient mode of operation. The solution is to actively control the transient behavior by controlling the temperature of the remote compensation chamber according to following rules:
T.sub.RCC+ΔT.sub.control=T.sub.SCC For one evaporator design ACHTL
T.sub.RCC+ΔT.sub.control=Max(T.sub.SCC1T.sub.SCC2, . . . T.sub.SCCn) for multiple n-evaporators design ACHTL

(18) This control will suppress all possible unwanted reverse flows and oscillations which can cause system failure. The selection of the parameter ΔT.sub.control is performed by modeling, considering most stressful transient scenarios of operation for ACHTL, such as maximum change and maximum ramp of input powers and condenser temperatures, orientation in gravity field, transport lines thermal environmental conditions, etc. During the test campaign the parameter ΔT.sub.control can be adjusted. Too large values of ΔT.sub.control can lead to unwanted degradation of ACHTL performance (lowered thermal conductance) in many nominal regimes of operation and finally to oversizing of the system since the subcooling is a function of condenser dimensions: higher subcooling needs larger condenser area. However, too small values of ΔT.sub.control can provoke ACHTL failure in transient modes. Typically, ΔT.sub.control lies in the range of 1÷10° C. To optimize the performance of the system the variable ΔT.sub.control as a function of ACHTL operational mode can be used. For instance: prior to startup event it is desirable to have large temperature differences between remote and stabilization compensation chambers (for instance, 5° C.) but after startup when all temperatures are stabilized it is possible to reduce ΔT.sub.control (for instance, 2° C.) to increase the performance of the ACHTL and reduce power consumption of the active control.

(19) The ACHTL device 1 can contain several evaporators 2 and several parallel condensers 24 even if in FIGS. 1a and 1b only two evaporators and one condenser are shown.

(20) The opportunity is provided that the evaporators 2 can collect the power from different heat sources, which could be located far one from the others thanks to the flexibility/adaptability provided by the ACHTL device 1 concept.

(21) The design of the volume of the stabilization-compensation chamber has to provide the possibility to cool and condense vapor bubbles generated by parasitic heat leak 12 (the chamber is functioning as a cold accumulator, providing effective compensation of the heat leak); to supply the liquid to primary capillary pump 4 (the chamber is functioning as a liquid accumulator, providing compensation of reduced liquid flow from condenser before the flow is fully developed and stabilized) in worst transient modes of ACHTL operation.

(22) A remote compensation chamber 20 (common for all evaporators 2 of multiple evaporator option) included in the proposed design serves to accumulate liquid and to compensate the liquid volume changes during the ACHTL device 1 operation. This large reservoir helps to avoid the obligation of designing a large volume compensation chamber for the individual evaporators in the multiple evaporator option. Therefore, this configuration allows to have a scalable design which can be fitted easier to the required number of evaporators and the specific requirements of each application, because evaporators will have the same design independently of the design and volume of the lines 18, 22, 23, condensers 24, total number of evaporators, etc. Only the volume of the remote compensation chamber 20 has to be adjusted for every specific ACHTL design.

(23) The advanced temperature control of the remote compensation chamber 20 can be realized in different ways, depending on each application requirements with the help of: a heater placed on the external surface of the remote compensation chamber (film type heater) a heater integrated into remote compensation chamber (cartridge type heater) a thermal electrical cooler placed on the external surface of the remote compensation chamber with the option to heat up or cool down by a change of voltage polarity

(24) Although the present invention has been fully described in connection with preferred embodiments, it is evident that modifications may be introduced within the scope thereof, not considering this as limited by these embodiments, but by the contents of the following claims.