THERMAL TESTS AND TWO-PHASE PHENOMENA OBSERVATIONS IN A TRANSPARENT EXPERIMENTAL LOOP HEAT PIPE

As the Loop Heat Pipe (LHP) technology becomes widely accepted for spacecraft thermal control systems, engineers are pushing the LHP operational envelope to design regimes that were not attempted before. As a result, a few so-called “anomalous phenomena” have been discovered, most of which occurred during ground testing. Temperature oscillation, notably on the liquid line, was reported as far back as 1999. At the time, the oscillation was generally deemed benign since it caused no ill effect on the LHP performance. Recently, however, results of LHP testing at the U.S. Naval Research Laboratory revealed that a pump deprime could happen following the appearance of the temperature oscillation, even when the measured loop pressure drop was far below the capillary limit of the primary wick. Within a certain operating regime, the temperature oscillation persisted with a fairly regular frequency and amplitude despite external conditions being kept constant. In an attempt to offer a sensible explanation for the aforementioned LHP oscillatory behaviors, a theory based upon the concept of stability of a nonlinear dynamical system is proposed in this paper. Below a critical value of the characteristic parameter μ, the LHP is able to reach and remain steady state. Above μ, its dynamical state undergoes a bifurcation to morph into a periodic but stable state. The underlying mechanism for the transformation is the mutual modulation of five synergistic processes taking place simultaneously in the loop: phase-change heat/mass transfer (i) in the capillary pump body, (ii) in the pump core, (iii) in the reservoir, (vi) in the condenser, and (v) fluid dynamics in the transport lines. The phenomenon of temperature oscillation is therefore dependent on the right combination of system components and the operating conditions.

Effects of non condensable gas in an ammonia loop heat pipe operating up to 125 °C

Investigations on effect of noncondensable gas in a loop heat pipe with flat evaporator on deprime

<div class="htmlview paragraph">This paper presents a theory that explains the low frequency, high amplitude temperature oscillations in loop heat pipe (LHP) operation. Temperature oscillations with amplitudes on the order of tens of Kelvin and periods on the order of hours have been observed in some LHPs during ambient testing. There is presently no satisfactory explanation for such a phenomenon in the literature. It is well-known that the operating temperature of an LHP with a single evaporator is governed by the compensation chamber (CC) temperature, which in turn is a function of the evaporator power, condenser sink temperature, and ambient temperature. As the operating condition changes, the CC temperature will change during the transient but eventually reach a new steady state. Under certain conditions, however, the CC temperature never reaches a true steady state, but instead displays an oscillatory behavior. The proposed theory explains why low frequency, high amplitude temperature oscillations may occur when the LHP has a large thermal mass attached to the evaporator, a low applied heat load, and a very cold sink. When this condition prevails, there are some complex interactions among the CC, condenser, thermal mass and ambient, which allows the thermal mass to modulate the constant applied heat load into an oscillatory heat input to the evaporator. The oscillatory evaporator power is the source of the temperature oscillation, which can be correlated to the vapor front movement in the condenser. The theory agrees very well with previously published test data. Effects of various parameters on the amplitude and frequency of the temperature oscillation are also discussed.</div>

Experimental research on the thermal performance of a flat evaporator loop heat pipe with a new composite wick

Influence of nanofluid on heat transfer in a loop heat pipe

In company with extreme developments of electronic devices, there are some unavoidable challenges to the conventional cooling methods such as high heat dissipation, limitation of cooling space, reliable operation as well as saving energy consumption. Therefore, the necessity of studying on new or how to improve the existing technologies is undoubted. Among various methodologies, the loop heat pipe (LHP) whose operation principle base on phase changing process, can be considered as one of the potential solutions of modern electronics cooling. This paper introduces the experimental investigation on the thermal performance of a flat-rectangular evaporator LHP with sintered stainless-steel wick when functioning under gravity assisted condition. Working fluid of this LHP was water. The present LHP could maintain stable operation in the range of heating power from 50 W to 520 W and keep the temperature on the heater’s top surface at 85oC, commonly recommended as the limitation temperature of electronics, when heating power reaches value 350 W (129.6 kW/m2). Besides, when turning the heater off, it took about 15 minutes for the LHP to cool the heating block from 102oC to 37oC. In addition, an assumption of the boiling heat transfer is introduced in this paper to explain the performance of evaporator at different heat flux conditions of the experiment.

<div class="htmlview paragraph">Loop Heat Pipes (LHPs) are passive two-phase heat transport devices that have been baselined for many spacecraft thermal management applications. The design life of a spacecraft can extend to 15 years or longer, thus requiring a robust thermal management system. Based on conventional aluminum/ammonia heat pipe experience, there exists a potential for the generation of noncondensible gas in LHPs over the spacecraft lifetime. In addition, some applications would have the LHP evaporator attached directly to spacecraft equipment having large thermal mass.</div> <div class="htmlview paragraph">To address the potential issues associated with LHP operation with noncondensible gas and large thermal mass attached to the evaporator, a test program was implemented to examine the effect of mass and gas on ammonia LHP performance. Many laboratory test programs for LHPs have heat delivered to the evaporator through light-weight aluminum heater blocks. In order to represent realistic applications, an LHP was tested with additional mass attached to the evaporator. To further simulate potential end-of-life LHP performance, noncondensible gas was also introduced into the loop. This paper presents the test results and the effects on LHP performance when tested with both large evaporator mass and substantial noncondensible gas.</div>

Investigation on the effect of thermoelectric cooler on LHP operation with non-condensable gas

This paper discusses the impact of diamond nanofluid on heat transfer characteristics in a Loop Heat Pipe (LHP). In this study, diamond nanoparticles in water with particle mass concentration ranged from 0% to 3% is considered as the operational fluid within the LHP. The experiments are carried out by manufacturing the LHP, in which the setup consists of a water tank with pump, a flat evaporator, condenser installed with two pieces of fans, two transportation lines (vapor and liquid lines), copper pipe sections for attachment of the thermocouples and power supply. The uniqueness of the current experimental setup is the vapor line of LHP which is made of transparent plastic tube to visualize the fluid flow patterns. The experimental results are verified by Finite Element (FE) simulation using a three-dimensional (3D) model based on the heat transfer by conduction where the LHP as a whole is modeled by assuming it as a conducting medium without taking into account the events occurring inside the LHP. The LHP performance is evaluated in terms of transient temperature distribution and total thermal resistance (Rt). The experimental and simulation results are found in good agreement.

Effect of Al2O3-H2O Nanofluid Concentration on Heat Transfer in a Loop Heat Pipe

Effect of tilt angle and base plate design on the performance of a loop heat pipe driven by vapor–liquid injector

Loop heat pipes (LHPs) are attractive devices to passively cool the electronic devices in spacecraft. The LHP operation needs to be shut off in some cases, for example, lunar rovers on cold nights, so that the temperature of the electric devices does not decrease too much. Some techniques have been developed for LHPs capable of heat switching in either active or passive ways. We develop a new active technology. To control liquid flow in the liquid line of the LHP, an electrohydrodynamic (EHD) conduction pump is placed in it. The EHD pump operates so that liquid flows in the reverse direction against the normal LHP flow. The pressure developed by the EHD pump overcomes the capillary pressure developed in an evaporator, and then the LHP operation stops. The advantages of using the EHD pump to control LHP operation are a low pressure drop during normal LHP operation, low power consumption, and no moving parts, although electric power and high voltage are needed. To demonstrate our proposal, an EHD conduction pump has been developed and evaluated to obtain pump characteristics with varying applied voltages. EHD simulations were conducted to design the pump, including the electrode configuration. Because EHD pumps can work only with dielectric liquids, R134a was chosen as the working fluid. When an appropriate high voltage was applied to the EHD pump incorporated into the liquid line of an LHP, the LHP was shut down successfully.

The loop heat pipe (LHP) was invented in Russia in the early 1980’s. It is a two-phase heat transfer device that utilizes the evaporation and condensation of a working fluid to transfer heat, and the capillary force developed in fine porous wicks to circulate the fluid. The temperature of LHP evaporator as functions of the heat load, sink temperature, ambient temperature is an important parameter which can reflect the performance of an LHP. Many factors can affect the LHP operating temperature and which can be divided into two parts: external and internal. The external factors including heat sink temperature, ambient temperature, fluid forces, the position between heat source and heat sink and the heat exchange between LHP and ambient. The internal factors related to the design and structure of the LHP, for example, the charging amount of the working fluid and the distribution status of the liquid phase during the LHP operating. Based on Sinda/Fluint software an ammonia-stainless steel steady state model of loop heat pipe was established, the impacts on the LHP operating temperature induced by alterable heat loads under 3 operating cases (the different position between evaporator and condenser, the changing of ambient temperature and the changing of heat sink temperature) were analyzed and conclusions were made. Changing the position between evaporator and condenser has a significant influence on the LHP operating temperature. Anti-gravity operation will reduce the performance of the LHP, this phenomenon is obviously in low heat load range. Further more, increasing of fluid pressure drop in the loop will induce decreasing of the LHP performance. The temperature difference between ambient and heat sink will influence the transition heat load (from variable conductance mode to fixed conductance mode), the bigger the temperature difference the higher the transition heat load.

In this paper is presented the results on performance of the cooling model using Loop Heat Pipe (LHP) system. In recent years, ever-ending demand of high performance CPU led to a rapid increase in the amount of heat dissipation. Consequently, thermal designing of electronic devices need to consider some suitable approach to achieve high cooling performance in limited space. Heat Pipe concept is expected to serve as an effective cooling system for laptop PC, however, it suffered from some problems as follows. The heat transport capability of conventional Heat Pipe decreases with the reduction in its diameter or increase in its length. Therefore, in order to use it as cooling system for future electronic devices, the above-mentioned limitations need to be removed. Because of the operating principle, the LHP system is capable of transferring larger amount of heat than conventional heat pipes. However, most of the LHP systems suffered from some problems like the necessity of installing check valves and reservoirs to avoid occurrence of counter flow. Therefore, we developed a simple LHP system to install it on electronic devices. Under the present experimental condition (the working fluid was water), by keeping the inside diameter of liquid and vapor line equal to 2mm, and the distance between evaporator and condenser equal to 200mm, it was possible to transport more than 85W of thermal energy. The thickness of evaporator was about 5mm although it included a structure to serve the purpose of controlling vapor flow direction inside it. Successful operation of this system at inclined position and its restart capability are confirmed experimentally. In order to make the internal water location visible, the present LHP system is reconstructed using transparent material. In addition, to estimate the limit of heat transport capability of the present LHP system using this thin evaporator, the air cooling system is replaced by liquid cooling one for condensing device. Then this transparent LHP system could transport more than 100W of thermal energy. However, the growth of bubbles in the reserve area with the increase in heat load observed experimentally led to an understanding that in order to achieve stable operation of the LHP system under high heat load condition, it is very much essential to keep enough water in the reserve area and avoid blocking the inlet with bubbles formation.