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.     

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.     

Thermal performance of screen mesh wick heat pipes using water-based copper nanofluids

Fairly stable surfactant free copper–distilled water nanofluids are prepared using prolonged sonication and homogenization. Thermal conductivity of the prepared nanofluid displays a maximum enhancement of ～15% for 0.5 wt% of Cu loading in distilled water at 30 °C. The wall temperature distributions and the thermal resistances between the evaporator and the condenser sections of a commercial screen mesh wick heat pipe containing nanofluids are investigated for three different angular position of the heat pipe. The results are compared with those for the same heat pipe with water as the working fluid. The wall temperatures of the heat pipes decrease along the test section from the evaporator section to the condenser section and increase with input power. The average evaporator wall temperatures of the heat pipe with nanofluids are much lower than those of the heat pipe with distilled water. The thermal resistance of the heat pipe using both distilled water and nanofluids is high at low heat loads and reduces rapidly to a minimum value as the applied heat load is increased. The thermal resistance of the vertically mounted heat pipe with 0.5 wt% of Cu–distilled water nanofluid is reduced by ～27%. The observed enhanced thermal performance is explained in light of the deposited Cu layer on the screen mesh wick in the evaporator section of the heat pipe.     

A three-dimensional thermal-fluid analysis of flat heat pipes

A detailed, three-dimensional model has been developed to analyze the thermal hydrodynamic behaviors of flat heat pipes without empirical correlations. The model accounts for the heat conduction in the wall, fluid flow in the vapor chambers and porous wicks, and the coupled heat and mass transfer at the liquid/vapor interface. The flat pipes with and without vertical wick columns in the vapor channel are intensively investigated in the model. Parametric effects, including evaporative heat input and size on the thermal and hydrodynamic behavior in the heat pipes, are investigated. The results show that, the vertical wick columns in the vapor core can improve the thermal and hydrodynamic performance of the heat pipes, including thermal resistance, capillary limit, wall temperature, pressure drop, and fluid velocities due to the enhancement of the fluid/heat mechanism form the bottom condenser to the top evaporator. The results predict that higher evaporative heat input improves the thermal and hydrodynamic performance of the heat pipe, and shortening the size of heat pipe degrades the thermal performance of the heat pipe.     

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.     

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.     

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.     

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.     

Thermal response of a flat heat pipe sandwich structure to a localized heat flux

The temperature distribution across a flat heat pipe sandwich structure, subjected to an intense localized thermal flux has been investigated both experimentally and computationally. The aluminum sandwich structure consisted of a pair of aluminum alloy face sheets, a truncated square honeycomb (cruciform) core, a nickel metal foam wick and distilled water as the working fluid. Heat was applied via a propane torch to the evaporator side of the flat heat pipe, while the condenser side was cooled via natural convective and radiative heat transfer. A novel method was developed to estimate experimentally, the heat flux distribution of the torch on the evaporator side. This heat flux distribution was modeled using a probability function and validated against the experimental data. Applying the estimated heat flux distribution as the surface boundary condition, a finite volume analysis was performed for the wall, wick and vapor core regions of the flat heat pipe to obtain the field variables in these domains. The results were found to agree well with the experimental data indicating the thermal spreading effect of the flat heat pipe.     

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.     

Theoretical and experimental investigation of a heat pipe heat exchanger for energy recovery of exhaust air

Heat pipes and two-phase thermosyphon systems are passive heat transfer systems that employ a two-phase cycle of a working fluid within a completely sealed system. Consequently, heat exchangers based on heat pipes have low thermal resistance and high effective thermal conductivity, which can reach up to the order of (10⁵ W/(m K)). In energy recovery systems where the two streams should be unmixed, such as air-conditioning systems of biological laboratories and operating rooms in hospitals, heat pipe heat exchangers (HPHEs) are recommended. In this study, an experimental and theoretical study was carried out on the thermal performance of an air-to-air HPHE filled with two refrigerants as working fluids, R22 and R407c. The heat pipe heat exchanger used was composed of two rows of copper heat pipes in a staggered manner, with 11 pipes per row. Tests were conducted at different airflow rates of 0.14, 0.18, and 0.22 m³/h, evaporator inlet-air temperatures of 40, 44, and 50°C, filling ratios of 45%, 70%, and 100%, and ratios of heat capacity rate of the evaporator to condenser sections (C_e/C_c) of 1 and 1.5. For HPHE's steady-state operation, a mathematical model for heat-transfer performance was set and solved using MATLAB. Results illustrated that the heat transfer rate was in direct proportion with the evaporator inlet-air temperature and flow rate. The highest HPHE's effectiveness was obtained at a 100% filling ratio and (C_e/C_c) of 1.5. The predicted and experimental values of condenser outlet-air temperature were in good agreement, with a maximum difference of 3%. HPHE's effectiveness was found to increase with the increase in evaporator inlet-air temperature and number of transfer units (NTU) and with the decrease in airflow rate, up to 33% and 20% for refrigerants R22 and R407c, respectively. Refrigerant R22 was the superior of the two refrigerants investigated.     

Analyzing the pressure-difference phenomenon between the condensing and boiling sections in a heat pipe cooling system

The heat pipe cooling system in this study consists of a flat evaporator, a condenser, and rising and falling tubes with water as working fluid. The working fluid has different water levels inside the two components. This is due to the vapor pressure deficits of the evaporation section and condenser section. This paper utilizes condensing and boiling pressure-difference theory and measures the temperature of the condenser wall to develop a theoretic model for the water level deficit inside the thermal module. Results indicate that the working fluid infiltrates the condenser and indirectly verifies the phenomenon leading to the different water levels inside the cooling system. Moreover, the water level height difference theory presented in this study may reduce the length of the condenser by 3.14 cm.     

Thermal performance characteristics of heat pipes

Theoretical and experimental studies are conducted to evaluate the overall thermal performance of single-component and gas-loaded heat pipes. In the analysis, the simple conduction model developed recently for the single-component heat pipes has been extended to predict the wall temperature profiles of gas-loaded heat pipes with phase change occurring in the evaporator wick. Experimental evaluation of the thermal performance is made with two working fluids (water and acetone) under two corresponding sink environments (boiling water and boiling alcohol). The heat pipe system is designed with variable-length heat input and output sections under a wide range of heat input conditions. Measured results agree well with theoretical predictions.     

Assessment of heat transfer and flow characteristics of a two-phase closed thermosiphon

In this study, comprehensive modeling and simulations were developed and carried out to perform the investigation of the thermal performance of the enclosed thermosiphon through pool boiling in the evaporator sector and the condensation of the liquid film in the condenser part. To simulate these phenomena, the volume of fluid model was utilized. The simulation modeling using the computational fluid dynamics (CFD) technique was validated with existing experimental results, and a good agreement was reached. The simulation results were presented and evaluated in terms of temperature profiles and contours, the volume of fraction contours, and velocity vector distribution. Moreover, the thermal performance (ie, the heat transfer coefficient and thermal resistance) through the thermosiphon operation was analyzed. From the simulation results, it is found that the thermosiphon performance can be improved by the tilt angle and fill ratio. The results indicated that the optimal performance (ie, a high heat transfer coefficient and a low thermal resistance) was attained at a power input of 250 W, tilt angle of 90°, and fill ratio of 0.5. The established CFD simulations effectively predicted the formation of two-phase flow pattern and boiling and condensation zones with water at a low power input, termed as geyser boiling.     

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.     

Performance degradation of flattened heat pipes

The performance degradation of flattened heat pipes is studied experimentally under a horizontal orientation. The original cylindrical copper/water heat pipes are ?6 mm and 30 cm in length. Tested are the sintered-powder wick and the groove wick. The maximum heat load (Q_max), the evaporator resistance (R_e), the condenser resistance, the overall thermal resistance, and the longitudinal temperature distributions are measured under incremented heat loads. After flattening, R_e is slightly reduced. Q_max is hardly affected when only the evaporator is flattened; but it is greatly reduced for fully flattened heat pipes. Different mechanisms of performance degradation are observed for flattened powdered and grooved heat pipes. With a thicker wick and larger saturate charge, the main degradation mechanism of flattened powdered heat pipes is liquid clogging at the condenser end. This causes malfunction of a powdered heat pipe flattened to 2.5 mm. When flattened to 3 mm, the powdered heat pipe exhibits milder Q_max degradation than a grooved heat pipe because the liquid flow is better protected against the vapor–liquid interfacial shear. In contrast, the serious Q_max degradation of a flattened grooved heat pipe is mainly caused by the interfacial shear which leads to greatly prompted dryout at the evaporator.     

A novel miniaturized loop heat pipe

The research on a novel miniaturized loop heat pipe (LHP) consisted of an evaporator, a condenser, vapor and liquid lines is presented in this paper. In the LHP, the evaporator was separated into two parts of boiling and suction chambers by a vapor separator, which drove vapor to one-way flow to vapor line. Moreover, the bottom of evaporator was connected as the cycle channel of refrigerant. Thin copper plates with micro-fins as enhanced structures fabricated by the ploughing–extrusion (P–E) method were embedded in the boiling chamber. Accordingly, the copper fiber sintered felt fabricated by the solid-phase sintering of copper fibers with rough surface, was filled in the suction chamber of evaporator as the wick to provide the capillary force. In addition, the integral rhombic-shaped pillars fabricated by the milling, behaved as intensified condensation structures in the condenser. The startup and operation characteristics of LHP were tested under different heat loads and refrigerants. The experimental results showed that the highest temperature of evaporator reached 93.2 °C under the maximum heat load of 150 W.     

Evaporative heat transfer model of a loop heat pipe with bidisperse wick structure

A mathematical model of evaporative heat transfer in a loop heat pipe was developed and compared with experiments. The steady-state thermal performance was predicted for different sintered nickel wicks, including monoporous and bidisperse structures. The effect of wick pore size distribution on heat transfer was taken into consideration. The wick in the evaporator was assumed to possess three regions during vaporization from an applied heat load: a vapor blanket, a two-phase region, and a saturated liquid region. The evaporator wall temperature and the total thermal resistance at different heat loads were predicted using ammonia as the working fluid. The predictions showed distinct heat transfer characteristics and higher performance for the bidisperse wick in contrast with monoporous wick. A bidisperse wick was able to decrease the thickness of the vapor blanket region, which presents a thermal resistance and causes lower heat transfer capacity of the evaporator. Additionally, a validation test presented good agreement with the experiments.     

Flower-type pulsating heat pipe for a solar collector

Inspired by the sunflower, we report a new structure of a solar collector that integrates a pulsating heat pipe (PHP) into a flat-plate collector. The proposed flower-type PHP solar collector is designed after a sunflower with petals that absorb sunlight and transfer nutrients to the stem after photosynthesis. The evaporator section adopts the shape of a flower to absorb sunlight fully, and the condenser section is rolled into a cylinder and placed in the lower part of the structure. A systematic experimental study is conducted upon start-up, and the performance characteristics, with acetone as the working fluid, are evaluated. We also did a heat loss analysis, which has a deviation of 8%. The effects of the mass flow rate of cooling water, filling ratio, length of the condenser section, and solar intensity are assessed. As the temperature of the heat absorber plate increases, the thermal resistance of the PHP can decrease to a minimum of 0.14°C/W. Under sunny weather conditions, the instantaneous thermal efficiency of the system with a filling ratio of 50% reaches 50%. Besides, we discussed the unstable operation conditions and possible dryout phenomenon that happened inside the PHP.     

Simulated and experimental performance of a heat pipe assisted solar wall

Performance was evaluated for a passive solar space heating system utilizing heat pipes to transfer heat through an insulated wall from an absorber outside the building to a storage tank inside the building. The one-directional, thermal diode heat transfer effect of heat pipes make them ideal for passive solar applications. Gains by the heat pipe are not lost during cloud cover or periods of low irradiation. Simplified thermal resistance-based computer models were constructed to simulate the performance of direct gain, indirect gain, and integrated heat pipe passive solar systems in four different climates. The heat pipe system provided significantly higher solar fractions than the other passive options in all climates, but was particularly advantageous in cold and cloudy climates. Parametric sensitivity was evaluated for material and design features related to the collector cover, absorber plate, heat pipe, and water storage tank to determine a combination providing good thermal performance with diminishing returns for incremental parametric improvements. Important parameters included a high transmittance glazing, a high performance absorber surface and large thermal storage capacity.An experimental model of the heat pipe passive solar wall was also tested in a laboratory setting. Experimental variations included fluid fill levels, addition of insulation on the adiabatic section of the heat pipe, and fins on the outside of the condenser section. Filling the heat pipe to 120% of the volume of the evaporator section and insulating the adiabatic section achieved a system efficiency of 85%. Addition of fins on the condenser of the heat pipe did not significantly enhance overall performance.The computer model was validated by simulating the laboratory experiments and comparing experimental and simulated data. Temperatures across the system were matched by adjusting the model conductances, which resulted in good agreement with the experiment.