Thermal Performance of a Closed-Loop Pulsating Heat Pipe With Multiple Heat Sources

A closed-loop pulsating heat pipe with multiple heat sources (CLPHP w/MHS) was invented to be used as a heat transfer medium between a number of heat sources to a single heat sink. However, an issue on the suitable heat source arrangement that causes the heat pipe to have the highest thermal performance was suspicious. The CLPHP w/MHS was made of a copper capillary tube with 32 turns. There were three heat sources with nonidentical input heat flux installed along a longitudinal axis in the evaporator section. Experimental investigations were conducted by permuting the heat sources into six unduplicated arrangements. For the vertical CLPHPs, the highest thermal performance is achieved when heat sources are arranged in consecutive order ascending from the lowest heat flux at the inlet of the evaporator section, since working fluid is promoted to circulate in complete one direction and then the heat can transfer more continuously. Finally, for the horizontal CLPHPs, the highest thermal performance is achieved when the heat sources are arranged in opposite order to the case of vertical CLPHPs, that is, descending from the highest heat flux, since working fluid pulsates with no intermission stop and this causes the heat transfer to be not interrupted.     

Experimental Investigation of Pulsating Heat Pipes and a Proposed Correlation

Pulsating heat pipes are complex heat transfer devices, and their optimum thermal performance is largely dependent on different parameters. In this paper, in order to investigate these parameters, first a closed-loop pulsating heat pipe (CLPHP) was designed and manufactured. The CLPHP was made of copper tubes with internal diameters of 1.8 mm. The lengths of the evaporator, adiabatic, and condenser sections were 60, 150, and 60 mm, respectively. Afterward, the effect of various parameters, including the working fluid (water and ethanol), the volumetric filling ratio (30%, 40%, 50%, 70%, 80%), and the input heat power (5 to 70 W), on the thermal performance of the CLPHP was investigated experimentally. The results showed that the manufactured CLPHP has the best thermal performance for water and ethanol as working fluids when the corresponding filling ratios are 40% and 50%, respectively. Finally, with the available experimental data set of CLPHPs, a power-law correlation based on dimensionless groups was established to predict their input heat flux. Compared with the experimental data, the root-mean-square deviation of the correlation prediction was 19.7%, and 88.6% of the deviations were within ± 30%.     

Heat transfer characteristics of the evaporator section using small helical coiled pipes in a looped heat pipe

An experimental study was carried out for the heat transfer characteristics and the flow patterns of the evaporator section using small diameter coiled pipes in a looped heat pipe (LHP). Two coiled pipes: the glass pipe and the stainless steel pipes were used as evaporator section in the LHP, respectively. Flow and heat transfer characteristics in the coiled tubes of the evaporator section were investigated under the different filling ratios and heat fluxes. The experimental results show that the combined effect of the evaporation of the thin liquid film, the disturbance caused by pulsation and the secondary flow enhanced greatly the heat transfer and the critical heat flux of the evaporator section. In final, two dimensionless empirical correlations were proposed for predicting the heat transfer coefficients of the evaporator section before and after dryout occurs.     

Correlation to predict heat transfer characteristics of a closed-end oscillating heat pipe at normal operating condition

This paper describes the effect of dimensionless parameters on the characteristics of heat transfer in a closed-end oscillating heat pipe (CEOHP). The parameters studied in this paper are; (i) Bond numbers, (ii) Froude numbers, (iii) Weber numbers, (iv) Prandtl numbers and (v) Kutateladze numbers. Experiments were conducted to find out their effects on the heat transfer rates of copper CEOHPs with inner diameters of 0.66, 1.06 and 2.03 mm. The lengths of the evaporator, adiabatic and condenser sections were equal and changed to 15, 10 and 5 cm. The total lengths of the CEOHPs were 15, 10 and 5 m. R123, ethanol and water were used as the working fluids with a filling ratio of 50%. The evaporator was heated by hot water, while the condenser section was cooled by a solution of water and ethylene glycol with a 1:1 mixing ratio by volume. The angle of the CEOHPs used in the experiments was set at 0° (horizontal mode) with a controlled vapor temperature of 50 °C. When the system reached the steady state, the temperature and the flow rate of the cooling substance was recorded in order to calculate the heat transfer rate of the CEOHP. It was found from the experiment that when R123 was used, the inner diameter affected the heat flux. The results of the experiment showed that when the inner diameter was larger, so was the heat flux. The result for ethanol showed the opposite; when the inner diameter increased, the heat flux decreased. Because of insufficient data obtain from using water as working fluid, the heat flux could not be reliably measured. The evaporator section lengths also affected the heat flux. The evaporator length of 15 cm gave the lowest value of heat flux. When the number of turns decreased, the heat flux increased and when n is equal to 14, heat flux is still increasing with an decrease in n. The results of the experiment also showed that the correlation equation could be used to predict the heat flux and that the operation map could predict the operational range and the inner diameter.     

Numerical model of a multi-turn Closed Loop Pulsating Heat Pipe: Effects of the local pressure losses due to meanderings

A numerical code has been developed to investigate the thermal performances of Closed Loop Pulsating Heat Pipes (CLPHP). The model takes into account the effects of the local pressure losses due to the presence of turns which have always been neglected by previous models; it can simulate CLPHPs working with different fluids (ethanol, R123 and FC-72 are shown), different number of turns, various inclination angles as well as different input heat fluxes at the evaporator. Numerical results show that the local pressure losses influence the device behavior in particular for high input heat fluxes and when the CLPHP is working in the horizontal position. The trends of the total liquid momentum, maximum tube temperatures, and equivalent thermal resistances, reveal good qualitative and quantitative accordance with the experimental data available in literature. Further direct experimental validations are mandatory to confirm whether this model can be used as a preliminary CLPHP thermal design tool.     

分离式热管倾斜蒸发段传热特性的试验研究

采用加热无缝钢管模拟倾斜布置的分离式热管蒸发段,分析了倾角、充液量、热流密度和工作压力对其传热特性的影响,并根据试验结果回归整理了相应的换热系数无量纲准则关系式,与试验数据吻合较好,计算误差小于15％。     

Correlation to predict heat transfer characteristics of a radially rotating heat pipe at vertical position

The heat transfer characteristics of a radially rotating heat pipe (RRHP) depend on a number of parameters. This paper is a study of the effects of these parameters. They are the inner diameter of the tube, aspect ratio, rotational acceleration, working fluid and the dimensionless parameters of heat transfer. RRHPs, made of copper tubes with inner diameters of 11, 26, and 50.4 mm, were used in the experiments. The aspect ratios were 5, 10, 20 and 40 respectively. The selected working fluids were water, ethanol and R123 (CHCl₂CF₃) with a filling ratio of 60% of evaporator volume. The experiments were conducted at inclination angles of 0–90° from horizontal axis and the rotational accelerations were lower, higher and equal to gravitational acceleration. The working temperature was 90 °C. The evaporator section was heated by electric power while heat in the condenser section was removed naturally by air. The evaporator and adiabatic section of the RRHP were well insulated with ceramic fibers. The experimental results showed that the heat flux decreases with an increasing inner diameter, and decreases with an increasing aspect ratio. The heat flux increases with an increasing rotational acceleration and decreases with an increasing liquid density of the working fluid. A correlation to predict the heat transfer rate at vertical position can be established.Further research will investigate a visual study of internal flow pattern and the formulation of a mathematical model.     

Predicting thermal instability in a closed loop pulsating heat pipe system

Mathematical models for a closed loop pulsating heat pipe (CLPHP) with multiple liquid slugs and vapor plugs are presented in this study. The model considers the effect of thermal instability in different sections of a CLPHP at different operational conditions. Based on a neural network, an approach of nonlinear autoregressive moving average model with exogenous inputs (NARMAX) can be applied to the thermal instability of CLPHP. This study approximates the nonlinear behavior of CLPHP with a linear approximation method that can establish the relationship among the response temperature differences between evaporator, adiabatic, and condenser sections. A multi-input single-output (MISO) strategy is adopted in this study to approximate nonlinear behavior of CLPHP. The predicted results show that the effect of the three sections to vapor condensation could be precisely distinguished; meanwhile, thermal performance of CLPHP would be predicted. The development of nonlinear identification technique will be helpful to optimize and understand the heat transfer performance of thermal instability in the different designs of CLPHP.     

Performance Prediction of Compact Fin-and-Tube Heat Exchangers in Maldistributed Airflow

An experimental study of a fin-and-tube heat exchanger was performed in two different configurations (single and three-screen mode). To this end, a test rig was constructed to evaluate the heat transfer capacity on the air side and water side. A wide range of Reynolds numbers on the air side was investigated. A series of measurements was performed with uniform inlet flow conditions. These served to determine the heat transfer correlation for the fin type using the Wilson plot method. No correlation was available, as the fin is an adapted inclined louvered type. To validate these results, a thorough uncertainty analysis was performed. Parallel to the experiments, a simulation program was written, designed to take non-uniform flow into account. The program is based on a local (section by section) analysis scheme. To validate the program, a series of non-uniform measurements was performed. Results showed that the program is able to predict the impact of non-uniform inlet flow conditions. The numerical code can be used as a design tool to develop more efficient heat exchangers.     

Experimental Performance of R-134a-Filled and Water-Filled Loop Heat Pipe Heat Exchangers

Experimental investigations were conducted to determine the thermal performances of an R-134a-filled thermosyphon heat pipe heat exchanger (THPHE) and a water-filled loop heat pipe heat exchanger (LHPHE) for hot and cold energy recovery for air conditioning purposes. For such applications, the heat pipe heat exchangers are operated at low temperatures. Both exchangers were operated in the countercurrent flow mode. This article presents the experimental results obtained. The results showed that heat transfer rate increased as evaporator inlet temperature increased and as both evaporator and condenser velocities increased. The overall effectiveness for the THPHE ranged from 0.8 to a minimum of about 0.5, while for the LHPHE it ranged from 0.9 to 0.3. Overall effectiveness was found to approach a minimum when both air streams have equal velocities.     

多通道并联回路型脉动热管运行特性的试验研究

针对回路型脉动热管进行了管路结构形式调整,制作了多通道并联回路型脉动热管并建立试验系统,选用丙酮和无水酒精作为工质,在相近热力工况下通过试验考察多通道并联回路型脉动热管和典型回路脉动热管在不同加热工况下的运行情况,并进行比较.结果表明:多通道并联回路型脉动热管与典型回路型脉动热管具有相似的启动特征,但其在运行中具有更好的稳定性,不易出现干烧现象;其传热效果也优于典型回路型脉动热管,具有较低的运行热阻,低充液率(34%)时的传热效果优于高充液率(51%、68%)时,具有较高的传热极限.     

Improvements of gravity assisted wickless heat pipes

The performance of conventional gravity assisted heat pipes and modified heat pipes with a separator in the adiabatic section is investigated experimentally. Heat pipes with a three layered wick in the evaporator section, in addition to the separator, are investigated. The performance of the modified heat pipes was compared to a reference gravity assisted heat pipe. Experiments were conducted on heat pipes of three lengths with a common diameter at constant evaporator and condenser lengths. The effect of varying the adiabatic length was, thus, investigated distinctly in normal heat pipes and in modified heat pipes with a separator. Water was employed as the working fluid in all heat pipes. The experimental program included five inclination angles and a heat flux range form 5 to 32 kW/m². The presence of the adiabatic separator caused a marked improvement in all heat pipes tested for all lengths and inclination angles. A pronounced reduction in heat pipe evaporator temperature was obtained, which is accompanied by an improvement in the heat transfer coefficient. A correlation was developed for prediction of the heat transfer coefficient for gravity assisted heat pipes with an adiabatic separator. The correlation took into consideration the effect of the varying adiabatic length. The correlation was in good agreement with the experimental data.     

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.     

Mathematical Modeling of Closed-End Pulsating Heat Pipes Operating with a Bottom Heat Mode

The mathematical model of a closed-end pulsating heat pipe (CEPHP) with a bottom heat mode at different inclination angles was constructed. The closed-end pulsating heat pipe was modeled with specified assumptions that were observed visually (i.e., the scaling factor for geometrical size and the frequency of bubble generation inside the liquid slugs). The solution for all of the basic governing equations of liquid film, liquid slugs, and vapor plugs, in which the effects of surface tension, viscous friction of the working fluid, and perfect gas were included, has been numerically obtained by solving a series of ordinary differential equations by means of the explicit method. However, the solution for the momentum equation of liquid slugs was numerically obtained by solving a series of partial differential equations by using the implicit method. Results from the model clearly simulated the dynamics of the internal working fluid in the CEPHP. Moreover, the results were compared with existing experimental data, and good agreement was found with an error range of ± 13%. It was also noted that the maximum heat transfer rate of the CEPHP with bottom heat mode occurred at the highest evaporator temperature (150°C for this study) and inclination angles of 70–80 degrees from horizontal axis. The boiling frequencies in this range of inclination angles were observed by visual experiment and seen to be at their highest values. This has been justified by the higher amount of liquid in the evaporator section as well as the change in flow pattern to a stratified flow (inclination tube).     

Heat transfer with flow and phase change in an evaporator of miniature flat plate capillary pumped loop

An overall two-dimensional numerical model of the miniature flat plate capillary pumped loop (CPL) evaporator is developed to describe the liquid and vapor flow, heat transfer and phase change in the porous wick structure, liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and metallic wall. The entire evaporator is solved with SIMPLE algorithm as a conjugate problem. The effect of heat conduction of metallic side wall on the performance of miniature flat plate CPL evaporator is analyzed, and side wall effect heat transfer limit is introduced to estimate the performance of evaporator. The shape and location of vapor-liquid interface inside the wick are calculated and the influences of applied heat flux, liquid subcooling, wick material and metallic wall material on the evaporator performance are investigated in detail. The numerical results obtained are useful for the miniature flat plate evaporator performance optimization and design of CPL.     

Enhancing the heat transfer performance of triangular-pitch shell-and-tube evaporators using an interior spray technique

In spray type evaporators using a conventional overhead spray method, a dry-out phenomenon occurs on the lower surface of the evaporator tubes under high surface heat flux conditions, and thus the heat transfer performance of the evaporator system is seriously impaired. This study shows that in a compact triangular-pitch shell-and-tube evaporator, the dry-out problem can be delayed through the use of an interior spray method, in which each heater tube within the bundle is sprayed simultaneously by two nozzles. The experimental results reveal that the shell-side heat transfer coefficients obtained using the proposed spray technique are significantly higher than those achieved in a conventional flooded type evaporator. The results also show that the heat transfer performance improves as the saturation temperature decreases since the density and thermal conductivity of the sprayed liquid increase. Finally, it is shown that for a constant heat flux and saturation temperature, the heat transfer coefficient increases with an increasing refrigerant mass flow rate.     

Flow Patterns and Heat Transfer for Flow Boiling in Small to Micro Diameter Tubes

An overview of the recent developments in the study of flow patterns and boiling heat transfer in small to micro diameter tubes is presented. The latest results of a long-term study of flow boiling of R134a in five vertical stainless-steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1, and 0.52 mm are then discussed. During these experiments, the mass flux was varied from 100 to 700 kg/m²s and the heat flux from as low as 1.6 to 135 kW/m². Five different pressures were studied, namely, 6, 8, 10, 12, and 14 bar. The flow regimes were observed at a glass section located directly at the exit of the heated test section. The range of diameters was chosen to investigate thresholds for macro, small, or micro tube characteristics. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapor quality for low quality values. At higher quality, the heat transfer coefficients decreased with increasing quality, indicating local transient dry-out, instead of increasing as expected in macro tubes. There was no significant difference between the characteristics and magnitude of the heat transfer coefficients in the 4.26 mm and 2.88 mm tubes but the coefficients in the 2.01 and 1.1 mm tubes were higher. Confined bubble flow was first observed in the 2.01 mm tube, which suggests that this size might be considered as a critical diameter to distinguish small from macro tubes. Further differences have now been observed in the 0.52 mm tube: A transitional wavy flow appeared over a significant range of quality/heat flux and dispersed flow was not observed. The heat transfer characteristics were also different from those in the larger tubes. The data fell into two groups that exhibited different influences of heat flux below and above a heat flux threshold. These differences, in both flow patterns and heat transfer, indicate a possible second change from small to micro behavior at diameters less than 1 mm for R134a.     

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.     

Heat Transfer and Flow Pattern Characteristics for HFE-7100 Within Microchannel Heat Sinks

This study investigates the heat transfer characteristics and flow pattern for the dielectric fluid HFE-7100 within multiport microchannel heat sinks with hydraulic diameters of 480 μm and 790 μm. The test results indicate that the heat transfer coefficient for the smaller channel is generally higher than that of the larger channel. It is found that the heat transfer coefficients are roughly independent of heat flux and vapor quality for a modest mass flux ranging from 200 to 400 kg m^?2 s^?1 at a channel size of 480 μm and there is a noticeable increase of heat transfer coefficient with heat flux for hydraulic diameters of 790 μm. The difference arises from flow pattern. However, for a smaller mass flux of 100 kg m^?2 s^?1, the presence of flow reversal at an elevated heat flux for hydraulic diameters of 480 μm led to an appreciable drop of heat transfer coefficient. For a larger channel size of 790 μm, though the flow reversal is not observed at a larger heat flux, some local early partial dryout still occurs to offset the heat flux contribution and results in an unconceivable influence of heat flux. The measured heat transfer coefficients for hydraulic diameters of 790 μm are well predicted by the Cooper correlation. However, the Cooper correlation considerably underpredicts the test data by 35–85% for hydraulic diameters of 480 μm. The influence of mass flux on the heat transfer coefficient is quite small for both channels.     

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.