Numerical Investigation of the Performance of a U-Shaped Pulsating Heat Pipe

In this research the performance of a U-shaped pulsating heat pipe (PHP) was investigated using numerical methods. This heat pipe consists of two sections: The evaporator is set at the two ends of the pipe, and the middle part of the pipe comprises the condenser section. This heat pipe is a type of open looped pulsating heat pipe. The governing equations are derived analytically from the continuity, momentum, and energy equations and are solved implicitly. In this model, considering the liquid mesh, the rate of convection and boiling heat transfer in the U-shaped PHP, which has not been investigated as of yet, are examined. The effect of the evaporator temperature on the pulse amplitude and frequency, rate of convection, and boiling heat transfer is also investigated. The results show that by increasing the evaporator temperature, due to the increase in pulse amplitude and frequency, the rate of heat transfer due to convection and boiling in the pipe will increase too. Furthermore, it is derived that by increasing the evaporator temperature, the share of boiling heat transfer will increase. In order to validate the results, the calculated heat transfer is compared to experimental and analytical results, and it is seen that the suggested model correctly predicts the rate of heat transfer within a precise range.     

Effect of Operating Parameters on the Heat Transfer and Liquid Film Thickness of Revolving Heat Pipe

This paper presents a study on the effects of operating parameters on the liquid film thickness and heat transfer of revolving heat pipe. The effects of speed, radius of rotation, evaporator and condenser temperatures, and mass of the working fluid are considered. Also, the effects of these parameters on the maximum heat transfer and minimum mass of the working fluid supplied to the heat pipe are considered. A simplified theoretical model is presented to estimate the heat transfer and the liquid film thickness. The theoretical model is used to determine the driven forces on the control volume. The system of equations associated with the heat pipe model is solved using the fourth-order Runge–Kutta method through a numerical code written in MATLAB. The results show that the heat transfer increases by decreasing the mass of the working fluid and increasing the temperature difference through the heat pipe. They also show that the liquid film thickness increases with the decrease in temperature difference and with increase in the mass of fluid. The maximum heat transfer increases with the increase in the rotation speed. The minimum mass of the working fluid supplied to the heat pipe increases with the increase in temperature difference and with the decrease in the rotation speed.     

Experimental investigation of cryogenic oscillating heat pipes

A novel cryogenic heat pipe, oscillating heat pipe (OHP), which consists of an 4 × 18.5 cm evaporator, a 6 × 18.5 cm condenser, and 10 cm length of adiabatic section, has been developed and experimental characterization conducted. Experimental results show that the maximum heat transport capability of the OHP reached 380 W with average temperature difference of 49 °C between the evaporator and condenser when the cryogenic OHP was charged with liquid nitrogen at 48% (v/v) and operated in a horizontal direction. The thermal resistance decreased from 0.256 to 0.112 while the heat load increased from 22.5 to 321.8 W. When the OHP was operated at a steady state and an incremental heat load was added to it, the OHP operation changed from a steady state to an unsteady state until a new steady state was reached. This process can be divided into three regions: (I) unsteady state; (II) transient state; and (III) new steady state. In the steady state, the amplitude of temperature change in the evaporator is smaller than that of the condenser while the temperature response keeps the same frequency both in the evaporator and the condenser. The experimental results also showed that the amplitude of temperature difference between the evaporator and the condenser decreased when the heat load increased.     

Heat Transfer Studies of a Heat Pipe

The present investigation reports a theoretical and experimental study of a wire screen heat pipe, the evaporator section of which is subjected to forced convective heating and the condenser section to natural convective cooling in air. The theoretical study deals with the development of an analytical model based on thermal resistance network approach. The model computes thermal resistances at the external surface of the evaporator and condenser as well as inside the heat pipe. A test rig has been developed to evaluate the thermal performance of the heat pipe. The effects of operating parameters (i.e., tilt angle of the heat pipe and heating fluid inlet temperature at the evaporator) have been experimentally studied. Experimental results have been used to compare the analytical model. The heat transfer coefficients predicted by the model at the external surface of the evaporator and condenser are reasonably in agreement with experimental results.     

Startup of a horizontal lithium-molybdenum heat pipe from a frozen state

Results of the simulation of the startup from a frozen state of a molybdenum heat pipe with lithium working fluid are presented and discussed. The 1.8-m-long heat pipe was tested in the horizontal position and had a liquid annular space between the porous wick and the wall. The 30-cm-long evaporator section was inductively heated and the 147-cm-long condenser was cooled by thermal radiation to the quartz tube enclosing the heat pipe and to the ambient. The space between the quartz tube and the heat pipe was evacuated in order to minimize heat losses by convection and conduction. Model results on the progression of the thaw front, liquid pooling at the end of the condenser, and the wall temperature along the heat pipe were found to be in good agreement with experimental measurements. Results showed that, as the heat pipe reached quasi-steady state operation at an evaporator wall temperature of 1550 K, the wall temperature near the end of the condenser dropped precipitously by 450 K, because of the formation of a 8.3-cm-long liquid plug and the end heat losses in the condenser.     

Flat Loop Heat Pipe with Bi-Transport Loops for Graphics Card Cooling

A flat loop heat pipe (FLHP) with bi-transport loops is developed for the cooling of graphics card with high heat flux up to 80W/cm². The evaporator and the pipes are made of copper and ultrapure water (electronic resistivity > 18 MΩ-cm) as the working fluid. To give the loop heat pipe (LHP) better performance, the evaporator is made in a flat shape to reduce the contact resistance between the evaporator and the chip. The advances of the LHP with bi-transport loops are discussed. The heat transfer performance is tested with different filling rate in different orientations. The test results show that the LHP can start up easily and can transport large amount of heat stably. The orientation of the condenser above the evaporator gives a better performance, and filling with 13 g of water gives a better performance. Limited by the evaporator temperature lower than 90°C, the LHP can transport 320 W when the evaporator is above the condenser and 380 W when the condenser is above the evaporator.     

Study on the heat-flow controllable heat exchanger (1st report): Thermal characteristics of heat-flow controllable heat exchanger

To regulate temperature in passive solar houses and green houses, the authors have developed a heat exchanger capable of controlling the heat flow. It has a thermal switch mechanism without mechanically moving parts. It consists of an evaporator, a condenser, a vapor passage pipe, a liquid return pipe having an inverted-U-pipe, and a heater mounted on the inverted-U-pipe. The heat exchanger can transfer, or reduce to zero, heat from the evaporator to the condenser by regulating a slight heater input. The authors have fabricated a model of the heat-flow controllable heat exchanger to examine its thermal switching and heat exchange characteristics, and then compared the obtained results with calculation results. It was clarified that the experimental results agree with the calculation results.     

Operational characteristics of oscillating heat pipes under micro-gravity condition

A mathematical model to investigate the oscillating motion characteristics of liquid slugs and vapor plugs/bubbles in oscillating heat pipes (OHPs) was developed considering the contact angle hysteresis (CAH) and interconnected-tube induced pressure fluctuations. Results show that a short period less than 1 s is available to attain the steady state after startup and then the oscillation amplitudes and frequencies for both of slug/bubble displacement and velocity are kept fixed. The slug/bubble displacement and velocity display quasi-sine oscillating waves with small pressure fluctuations induced by the interconnected-tube. However, small oscillation waves are superimposed on a main quasi-sine oscillation wave and cause a chaotic oscillating behavior of slug/bubble inside the OHP if the induced pressure fluctuation is large enough. Besides, the effects of filling ratio, tube length, inner diameter, temperature difference between the evaporator and condenser sections, and working fluid on the oscillating motion were numerically analyzed and discussed. The numerical model provides a physical insight to understand the operational mechanism of OHPs under the microgravity condition.     

Evaporation and condensation heat transfer in a heat pipe with a sintered-grooved composite wick

A mathematical model of evaporation and condensation heat transfer in a copper-water wicked heat pipe with a sintered-grooved composite wick is developed and compared with experiments. The wall temperatures are measured under different input power levels and working temperature conditions. The results show that the heat transfer in the condenser section was found to be only by conduction. In the evaporator, however, either conduction or boiling heat transfer can occur. The experimental data for the boiling heat transfer are well correlated by the theory of Stralen and Cole. Higher heat load drives the heat pipe to spend more time achieving the equilibrium state during the transient start-up process. The response curves of the evaporator thermal resistance are overlapped, and the condenser thermal resistance increases more sharply at the beginning. The total thermal resistance of the heat pipe ranges from 0.02 to 0.56 K/W.     

Passive downward heat transport: experimental results of a technical unit

A bench scale model for passive downward transport of heat has been built and tested. The heat is transported by evaporation of a fluid in an evaporator at a higher level and condensation at a lower level. The condensate is returned to the evaporator by the periodic operation of a self-actuated float valve without disconnectding the heat delivery to the evaporator. The cost of lifting the liquid back to the evaporator is a temperature difference of a few degrees centigrade between the evaporator and the heat store. The unit works under moderate pressure (a maximum of 9 bar at 85°C). The actual vertical distance between heat source and heat store is about 1 m; a pressure difference equivalent to a vertical distance of 15 m was introduced by two spring-loaded check valves; in these conditions the ΔT between heat source and heat store is about 10°C. This ΔT is comparable to that occurring in the condenser to promote heat transfer to the heat store medium. Most of the components of the 1000 W model are similar to those used in conventional refrigerators or heat pumps. The system can easily be integrated with a solar heat collector working with a 2-phase fluid.     

Mathematical modeling of steady-state operation of a loop heat pipe

A steady-state mathematical model of a loop heat pipe is established and compared with experimental results in this work. The modeling of the evaporator wick includes not only the single-layer wick, but also the two-layer compound wick. The annular flow model is adopted in the modeling of the condenser, in which the effect of surface tension of liquid and the interaction between the liquid and vapor phases including both frictional and momentum-transfer shear stresses are considered. The model can predict the decreasing length of the condenser two-phase zone under the constant conductance mode caused by the volume expansion of the liquid in the compensation chamber, and is in good agreement with the experimental data. It also shows that the application of the two-layer compound wick can improve the performance of the loop heat pipe operating under the variable conductance mode, due to the reduction of heat leak from the evaporator to the compensation chamber. A parametric study of the effect of heat sink temperature, ambient temperature, adverse elevation, and working fluid inventory on the operating temperatures of the loop heat pipe is also conducted, which further contributes to the understanding of the steady-state operating characteristics of loop heat pipes.     

Heat transfer in the evaporator section of moderate-speed rotating heat pipes

Experiments were performed to investigate the heat transfer mechanism in the evaporator section of non-stepped rotating heat pipes at moderate rotational speeds of 2000–4000 rpm or accelerations of 40g–180g, and evaporator heat fluxes up to 100 kW/m². The thermal resistance of the evaporator section as well as that of the condenser section was examined by measuring the axial temperature distributions of the flow in the core region of the heat pipe and along the wall of the heat pipe. The experimental results indicated that natural convection heat transfer occurred in the liquid layer of the evaporator section under these conditions. The heat transfer measurements were in reasonable agreement with the predictions from an existing rotating heat pipe model that took into account the effect of natural convection in the evaporator section.     

Investigation of the Startup Condition of a Closed-Loop Oscillating Heat Pipe

This article develops a concept for a suitable startup condition for a closed-loop oscillating heat pipe (CLOHP). This concept was developed by using visual data and the thermodynamics theory for predicting the amount of vapor evaporation and condensation in a CLOHP. The visual data indicated that the key to a suitable startup is the amount of net vapor expansion in the evaporator and the amount of net collapsed vapor in the condenser. Initial dryout, an event that occurs after a startup failure, results when the net vapor expansion is higher than the amount of net vapor collapsed. This situation obstructs the replacement process. This is a mechanism in which the volume of mixture from the condenser section flows to the evaporator section to replace the volume of mixture that leaves the evaporator section. When the replacement process is impeded, all of the liquid in the evaporator section evaporates and the evaporator section is not refilled by the mixture from the condenser section. The evaporator section is then filled with vapor and initial dryout occurs. In addition, this article presents a mathematical model that predicts the operating temperature for a suitable startup condition. This prediction can be used to avoid a startup failure of a CLOHP. When comparing the model with that of the experimental data, a 16% error range was attained.     

Analytical study of the heat transfer limits of a novel loop heat pipe system

A novel loop heat pipe system was designed for use in solar hot water heating and an analytical model was developed to investigate its thermal performance and determine six major limits to system operation, i.e. capillary limit, entrainment limit, viscous limit, boiling limit, sonic limit, and filled liquid mass limit. Relations among the limits and several associated parameters, i.e. the heat pipe operating temperature, wicks type, heat pipe diameter, and height difference between the absorbing pipes array and condenser (heat exchanger), were established through a comprehensive analyses. It was found that the levels of capillary, entrainment, viscous, sonic, and filled liquid mass limits increased with the increasing temperature; however, the boiling limit was in the adverse trend. It was also found that the mesh screen wicks were able to obtain a higher capillary limit than sintered powder wicks, whilst other limits remained same. Larger pipe diameters would lead to higher operating limits. The height difference between the condenser (heat exchanger) and absorbing pipes (absorber) was the most important factor impacting on heat transfer capacities of the system, and largely affected the capillary limit of the system. It was noted when the pipe (inner) diameter increased to 5.6 mm or above, the governing limit of the system switched from entrainment to capillary. Relationship between the system governing limit, i.e. capillary limit, and the above addressed parameters were analysed. Adequate system configuration and operating conditions were suggested, which were summarized as follows: 6 mm of pipe inner diameter with mesh screen wicks, 58°C of heat pipe operating temperature, and 1.3 m height difference between absorber and condenser (heat exchanger). Copyright © 2010 John Wiley & Sons, Ltd.     

A quasi-3D analysis of the thermal performance of a flat heat pipe

The thermal performance of a flat heat pipe thermal spreader has been described by a quasi-3D mathematical model and numerically modeled. An explicit finite volume method with under-relaxation was used for computations in the vapor phase. This was combined with a relatively small time step for the analysis. The physical problem consisted of an evaporator surface that was transiently heated non-uniformly for a short period of time and the heat source then removed. Then the system was cooled by natural convection and radiative heat transfer at the condenser region. The transient temperature distributions at the front and back of the heat spreader were obtained for different times during the transient period. The velocity distribution in the vapor core was also obtained. Due to the effect of phase change at the evaporator and condenser sides, a significant amount of energy is found to be absorbed and partially released during the transient heating and cooling processes. The numerical results indicate that advection and the high thermal diffusivity of the vapor phase accelerate the propagation of the temperature distribution in the vapor core, making it uniform during this process. The condenser temperature distribution was almost uniform at the end of the transient heating process. The transient temperature distribution on a solid aluminum plate was compared with the flat heat pipe results and indicated that the flat heat pipe successfully spread the heat uniformly at the condenser side of the structure.     

Heat pipe heat exchanger for heat recovery in air conditioning

The heat pipe heat exchangers are used in heat recovery applications to cool the incoming fresh air in air conditioning applications. Two streams of fresh and return air have been connected with heat pipe heat exchanger to investigate the thermal performance and effectiveness of heat recovery system. Ratios of mass flow rate between return and fresh air of 1, 1.5 and 2.3 have been adapted to validate the heat transfer and the temperature change of fresh air. Fresh air inlet temperature of 32–40 °C has been controlled, while the inlet return air temperature is kept constant at about 26 °C. The results showed that the temperature changes of fresh and return air are increased with the increase of inlet temperature of fresh air. The effectiveness and heat transfer for both evaporator and condenser sections are also increased to about 48%, when the inlet fresh air temperature is increased to 40 °C. The effect of mass flow rate ratio on effectiveness is positive for evaporator side and negative for condenser side. The enthalpy ratio between the heat recovery and conventional air mixing is increased to about 85% with increasing fresh air inlet temperature. The optimum effectiveness of heat pipe heat exchanger is estimated and compared with the present experimental data. The results showed that the effectiveness is close to the optimum effectiveness at fresh air inlet temperature near the fluid operating temperature of heat pipes.     

A study on the new separate heat pipe refrigerator and heat pump

A new separate heat pipe refrigerator and heat pump is suggested based on the general three temperature thermal jet refrigerator and heat pump cycle. Sub-cooled hot water or other appropriate liquid heated by low grade heat sources forms the hot end and another heat pipe containing evaporator and condenser ends, adiabatic section of two-phase ejector and throttling tube is as the cold end of the separate heat pipe system. Performance relations for the thermal jet refrigerator and heat pump of such system is analyzed and a method of thermodynamic performance analysis is recommended. Primary prediction shows the feasibility of such heat pipe system for cold and warm water supply.     

Heat transfer in a pulsating heat pipe with open end

Heat transfer in the evaporator and condenser sections of a pulsating heat pipe (PHP) with open end is modeled by analyzing thin film evaporation and condensation. The heat transfer solutions are applied to the thermal model of the pulsating heat pipe and a parametric study was performed. The results show that the heat transfer in a PHP is mainly due to the exchange of sensible heat. The frequency and amplitude of the oscillation is almost unaffected by surface tension after steady oscillation has been established. The amplitude of oscillation decreases with decreasing diameter. The amplitude of oscillation also decreases when the wall temperature of the heating section is decreased, but the frequency of oscillation is almost unchanged.     

Startup characteristics of a vertically placed high‐temperature heat pipe fin

In order to observe startup characteristics, a vertically installed high‐temperature heat pipe fin was tested. The temperature curves during the startup process are given. It was found that the evaporator bottom temperature in the high‐temperature heat pipe fin with a constant heat input increased very quickly over time. The temperature at the evaporator top and the condenser temperature lagged behind the temperature of the evaporator bottom. The evaporator outlet temperature coincided with the condenser middle temperature. The temperature at the end of the condenser exhibited a phenomenon of temperature pulsation. If the high‐temperature heat pipe fin was placed horizontally for a certain period of time and then tested in its vertical position, the temperature pulsation phenomenon at the condenser disappeared and a good isothermal condition emerged. Further analysis showed that larger heat inputs yielded faster startups and weaker pulsation during the startup period. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(6): 411–416, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20022     

冷库节能运行评价研究

冷库作为高能耗建筑应在节能运行方面进行考核,文章从传热学角度提出以冷凝器侧传热温差、蒸发器侧传热温差、围护结构热流量方面对冷库制冷系统性能和围护结构保温性能作考核,并运用此方法对冷库进行了抽检,作出了诊断和评价。