Performance of a R-134a-filled thermosyphon

Heat pipes are low cost and efficient heat exchange equipment. They are suitable for low temperature heat or cold recovery systems. The latter could be employed to cool incoming warm fresh air in air-conditioned ventilation systems. R-134a is an environmentally friendly refrigerant and has been generally accepted as a substitute for R-12 and R-22. The thermal performance of a thermosyphon filled with R-134a was investigated. The effects of temperature difference between bath and condenser section, fill ratio and coolant mass flow rates on the performance of the thermosyphon were determined. The experimental results indicate that the heat flux transferred increased with increasing coolant mass flow rate, fill ratio and temperature difference between bath and condenser section.     

Heat transfer characteristics of a two-phase closed thermosyphon to the fill charge ratio

In this study, the heat transfer characteristics of a two-phase closed thermosyphon were investigated. For the test, a two-phase closed thermosyphon (copper container, FC-72 (C₆F₁₄) working fluid) was fabricated with a reservoir which could change the fill charge ratio. The experiments were performed in the range of 50-600 W heat flow rate and 10-70% fill charge ratio. Some findings are as follows.The heat transfer coefficients of the evaporator to the fill charge ratio were nearly negligible. These presented about 1-5 kW/m² K with the increase of heat flux and compared with those of smooth surface, showed some enhancement by the grooved surface. However at the condenser, the heat transfer coefficients showed some enhancement with the increase of fill charge ratio by the expanded working fluid pool. And the heat transport limitations appeared in different ways to the fill charge ratio. For the relatively small fill charge ratio (Ψ<20%), it presented about 100 W (Ψ: 10%) by the dry-out limitation.For the large fill charge ratio, it occurred by the flooding limitation and the maximum heat flow rate was about 500-550 W (Bo: 26-28), 230 W (Bo: 18.3) respectively and the Kutateladze number was about 1.9-2.1.     

Thermal characteristics of a resurfaced condenser and evaporator closed two-phase thermosyphon

This paper reports a study on the effect of the condenser and evaporator resurfacing on overall performance of a 1 m height closed two-phase thermosyphon. Water was used as working fluid with a fill ratio and operating pressure was 0.75 and 160 mbar, respectively. The thermosyphon performances for plain and modified thermosyphon were studied at 44 power inputs from 43 W to 668 W. The results show that by making the evaporator more hydrophilic and the condenser more hydrophobic, it will be possible to increase the average thermal performance by15.27% and decrease the thermal resistance by 2.35 times compared with the plain one.     

Experimental investigation of heat transfer coefficient of R134a during condensation in vertical downward flow at high mass flux in a smooth tube

The two-phase heat transfer coefficients of pure HFC-134a condensing inside a smooth tube-in-tube heat exchanger are experimentally investigated. The test section is a 0.5 m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is constructed from smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The test runs are performed at average saturation condensing temperatures between 40–50 °C. The mass fluxes are between 260 and 515 kg m^− 2s^− 1 and the heat fluxes are between 11.3 and 55.3 kW m^− 2. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy transferred from the test section. The effects of heat flux, mass flux and condensation temperature on the heat transfer coefficients are also discussed. Eleven well-known correlations for annular flow are compared to each other using a large amount of data obtained from various experimental conditions. A new correlation for the condensation heat transfer coefficient is proposed for practical applications.     

微型螺旋管蒸发器的沸腾换热与阻力特性实验研究

设计、制造并测试了一种微型螺旋管蒸发器。对微型螺旋管蒸发器的沸腾换热与阻力特性进行了实验研究。根据实验数据获得雷诺数在1460~10000范围内流阻系数与雷诺数的计算关系式。实验结果表明,这种微型蒸发器换热量超过300W。     

Experimental analysis for the determination of the convective heat transfer coefficient by measuring pressure drop directly during annular condensation flow of R134a in a vertical smooth tube

This study investigated the direct relationship between the measured condensation pressure drop and convective heat transfer coefficient of R134a flowing downward inside a vertical smooth copper tube having an inner diameter of 8.1 mm and a length of 500 mm during annular flow. R134a and water were used as working fluids on the tube side and annular side of a double tube heat exchanger, respectively. Condensation experiments were performed at mass fluxes of 260, 300, 340, 400, 456 and 515 kg m⁻² s⁻¹ in the high mass flux region of R134a. The condensing temperatures were around 40 and 50 °C; the heat fluxes were between 10.16 and 66.61 kW m⁻². Paliwoda’s analysis, which focused mainly on the determination of the two-phase flow factor and two-phase length of evaporators and condensers, was adapted to the in-tube condensation phenomena in the test section to determine the condensation heat transfer coefficient, heat flux, two-phase length and pressure drop experimentally by means of a large number of data points obtained under various experimental conditions.     

Performance investigation of nanofluids as working fluid in a thermosyphon air preheater

In recent years, there has been a substantial increase in energy demand due to industrialization development. This raises concern on issues such as depletion of fossil based energy and emission of green house gasses. Hence, optimization of energy use through the thermosyphon air preheater is one of the possible approaches to address this problem. It can be used to recover and transmit the heat from the hot air (flue gas) to the cold air used for combustion process in a boiler. This study focuses on the analytical analysis of the thermal performance of a thermosyphon operated with water and nanofluids. The thermo physical properties of the selected nanofluids and relevant formulations are taken from the literatures to perform the analysis. Study found that change of nanofluid properties such as thermal conductivity only plays minor role in enhancing the thermal performance of the thermosyphon. The study implied that the hot air velocity is capable of increasing the efficiency of a thermosyphon. It is found that 23% overall heat transfer enhancement is observed when the hot air velocity increases from 2.0 m/s to 4.75 m/s for water based (7%) alumina and (4%) titanium dioxide nanofluids.     

CFD modeling of flow and heat transfer in a thermosyphon

In the present study a gas/liquid two-phase flow and the simultaneous evaporation and condensation phenomena in a thermosyphon was modeled. The volume of fluid (VOF) technique was used to model the interaction between these phases. Experiments in a thermosyphon were carried out at different operating conditions. The CFD predicted temperature profile in the thermosyphon was compared with experimental measurements and a good agreement was observed. It was concluded that CFD is a useful tool to model and explain the complex flow and heat transfer in a thermosyphon.     

An experimental study on thermal mixing in a square body inserted inclined narrow channels

An experimental study has been performed on thermal mixing phenomena in a narrow channel by twin-jets at different temperatures. Water was used as working fluid and it is supplied by hot and cold taps. The channel has a circular exit hole to supply continuity of mass. An adiabatic square shaped object, which in the thickness of the channel, is inserted into the channel to control thermal mixing as a passive technique. Other parameters in experiments are ratio of flow rate of inlet fluid, inclination angle of the channel, jet diameter and jet velocities. Finally, a thermal mixing index was calculated from measured values of temperatures for different parameters. Temperature distribution is obtained for whole channel and isotherms are plotted. The obtained results indicated that higher thermal mixing efficiency is observed for ? = 60^o and inserted body can be a control parameter for thermal mixing for the same geometrical parameters.     

Effect of a varying effective thermal conductivity term on heat conduction through a physical model of a hydride bed

This paper investigates the effect of implementing a varying effective thermal conductivity term on heat transfer simulations within a metal hydride (MH) reactor. The work presented utilises a 1D transient heat transfer analysis comparing simulations either with a fixed effective thermal conductivity or a varying effective thermal conductivity. This builds on previously published work in the literature that has investigated varying effective thermal conductivity for metal hydrides at a single point. The results of these simulations are compared to experimental work for validation.     

New proposed two-phase multiplier and evaporation heat transfer coefficient correlations for R134a flowing at low mass flux in a multiport minichannel

New correlations of the two-phase multiplier and heat transfer coefficient of R134a during evaporation in a multiport minichannel at low mass flux are proposed. The experimental results were obtained from a test using a counter-flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Test section is composed of the extruded multiport aluminium inner tube with an internal hydraulic diameter of 1.2 mm and an acrylic outer tube with an internal hydraulic diameter of 25.4 mm. The experiments were performed at heat fluxes between 10 and 35 kW/m², and a refrigerant mass flux between 45 and 155 kg/(m² s). Some physical parameters that influenced the frictional pressure drop and heat transfer coefficient are examined and discussed in detail. The pressure drop and heat transfer coefficient results are also compared with existing correlations. Finally, new correlations for predicting the frictional pressure drop and heat transfer coefficient at low mass fluxes are proposed.     

Study of flow boiling characteristics of R134a in annulus of enhanced surface tubing

The paper presents an experimental study of the flow-boiling heat-transfer characteristics of R12 and R134a in the annulus of a horizontal enhanced-surface-tubing evaporator. The test section has an inner-tube bore diameter of 17.5 mm, an envelope diameter of 28.6 mm and an outer smooth tube of 32.3 mm inside diameter. The ranges of heat flux and mass velocity covered in the tests were 5–25 kW/m² and 180–290 kg/m²/s, respectively, at a pressure of 365 kPa. In order to establish the flow regime conditions at the inlet to the test section, the test rig allows for the visualization of refrigerant flow through the preheater. The experiments show two regions of heat transfer: a nucleate boiling region where the heat transfer depends mainly on heat flux, and a forced convective region where the heat transfer depends only on the refrigerant flow rate.     

Experimental investigation of the transient thermal performance of a bent heat pipe with grooved surface

A bent copper–water heat pipe with grooved inner surface has been investigated experimentally. A comparison between the bent and the straight heat pipes was performed at different inclination angle. Experimental results show that there is a small temperature difference between the condenser of the straight and that of the bent at the vertical orientation. The temperature difference increases as an inclination angle increases. Furthermore, the response time increases as the inclination angle increases. The thermal response of the straight to a sudden heat load is slightly faster than that of the bent. However, as the inclination angle increases to after the horizontal, the heat flux at the condensers decreases nonlinearly and the response time increases nonlinearly. A two-phase flow map has been proposed to explain the nonlinear performance of the thermal response and the heat flux, based on force balance among gravity, capillary, friction and buoyancy force acting on the working fluids. The nonlinear performance of the thermal response and the heat flux results from the capillary blocking due to formation of liquid bridge of two-phase flow. It was also found that the bent heat pipe is more sensitive to the change of the inclination angle than the straight in terms of the thermal response time and the heat flux of the condenser. The heat flux of the bent decreases faster than that of the straight after the horizontal orientation.     

An experimental investigation into the performance of a domestic thermosyphon solar water heater under varying operating conditions

An experimental investigation has been carried out on a thermosyphon solar water heater. The system consisted of a flat-plate collector of 1.5 m² absorber area with 21 tubes/m width and storage tank of 125 litre capacity. Experiments were carried out for both cloudy and clear weather conditions in winter and summer. The hourly system performance was evaluated for all test conditions. The final mean tank temperature was measured daily which enabled the calculation of the possible contribution of solar energy for domestic hot water supply in Basrah, Iraq (latitude 30.76°N). The system was tested at both no-load and loading conditions. Intermittent and continuous load was imposed, and system performance was evaluated for each condition.     

Thermal performance of an open thermosyphon using nanofluids for high-temperature evacuated tubular solar collectors: Part 1: Indoor experiment

An especial open thermosyphon device used in high-temperature evacuated tubular solar collectors was designed. The indoor experimental research was carried out to investigate the thermal performance of the open thermosyphon using respectively the deionized water and water-based CuO nanofluids as the working liquid. Effects of filling rate, kind of the base fluid, nanoparticle mass concentration and the operating temperature on the evaporating heat transfer characteristics in the open thermosyphon were investigated and discussed. Experiment results show the optimal filling ratio to the evaporator is 60% and the thermal performance of the open thermosyphon increase generally with the increase of the operating temperature. Substituting water-based CuO nanofluids for water as the working fluid can significantly enhance the thermal performance of the evaporator and evaporating heat transfer coefficients may increase by about 30% compared with those of deionized water. The CuO nanoparticles mass concentration has remarkable influence on the heat transfer coefficient in the evaporation section and the mass concentration of 1.2% corresponds to the optimal heat transfer enhancement.     

Experimental study on the condensation of a thermosyphon by electrical capacitance tomography

The study on the condensation and the two-phase flow pattern in the condensation section is important to understand the operating mechanisms in a thermosyphon. In this paper, a new electric capacitance tomography (ECT) sensor was designed for the visualization measurement in a liquid by removing the shielding case and sealing with insulating hydrophobic material. It was successfully used to measure the condensation process in a thermosyphon under different operating temperatures. The thermosyphon was made of silica glass, and alcohol was used as a working fluid. The alcohol vapor was cooled to condense through the heat convection with the cooling water. The operating temperature was controlled by a heater with different power outputs. The experimental results show that the alcohol vapor condensed in stripes and unevenly on the wall surface at a low operating temperature. The liquid bridge will be formed periodically at the operating temperature of 90°C, and the time interval between two liquid bridges will be shorter with the increase of the operating temperature. At 117°C or even higher operating temperatures, the complete liquid bridge cannot be formed sometimes due to the difference of the growth rate of the surface wave around the circumference.     

Experimental and analytical performance study of a thermosyphon water heater

The performance of a thermosyphon solar water heater was studied analytically and experimentally. A finite-difference model was used to predict year-round performance. Tests were conducted on an experimental heater subjected to acutal weather conditions in Benghazi, Libya. Satisfactory qualitative and quantitative agreement was found between experimental and predicted results. A storage volume of 60 liters per unit collector area was found to be optimum for Benghazi conditions. The day-end temperature was found to vary between 23° and 51°C for the test period, which occurred in winter, with an average of 41°C.     

Effect of void fraction models on the film thickness of R134a during downward condensation in a vertical smooth tube

In the present study, the void fraction and film thickness of pure R-134a flowing downwards in a vertical condenser tube are indirectly determined using relevant measured data together with an annular flow model and various void fraction models reported in the open literature. The vertical test section is a countercurrent flow double tube heat exchanger with refrigerant flowing down in the inner tube and cooling water flowing upward in the annulus. The inner tube is made from smooth copper tubing of 9.52 mm outer diameter with a length of 0.5 m. The experimental runs are carried out at average saturated condensing temperatures of 40 and 50 °C, and mass velocities are around 456 kg m^− 2 s^− 1, over the vapour quality range 0.82–0.93, while the heat fluxes are between 45.60 and 50.90 kW m^− 2. Analysis based on simple void fraction models of the annular flow pattern are presented for forced convection condensation of pure R134a, taking into account the effect of the different saturation temperatures at high mass flux conditions. The comparisons of calculated film thickness show that the void fraction models of Spedding and Chen, and Chisholm and Armand are the most accurate ones with the experimental data due to their low deviation with Whalley's annular flow model over 35 void fraction models presented in this paper.     

Effect of void fraction models on the two-phase friction factor of R134a during condensation in vertical downward flow in a smooth tube

The void fraction of R134a condensing inside a vertical smooth tube is experimentally investigated in this study. The vertical test section is a 0.5 m long countercurrent flow double tube heat exchanger with refrigerant flowing down the inner tube and cooling water flowing upward in the annulus. The inner tube is made from smooth copper tubing and has an 8.1 mm inner diameter. The test runs are done at an average saturated condensing temperature of 40 °C. The average qualities are between 0.8 and 0.99 while the mass fluxes are 300 kg m^− 2s^− 1 and the heat fluxes are between 22 and 39.8 kW m^− 2. The void fractions are indirectly determined using relevant measured data together with various void fraction models and correlations reported in the open literature. The friction factors obtained from various void fraction models and correlations are compared with each other and also with those determined from graphical information provided by Bergelin et al. The effect of void fraction alteration on the momentum pressure drop is also presented.     

Comparative study on evaporator heat transfer characteristics of revolving heat pipes filled with R134a, R22 and R410A

Heat pipes are used extensively in various applications including the heating, ventilating and air conditioning (HVAC) systems. The high thermal conductivity of the device, attributed from the two-phase heat transfer processes within the heat pipe, made them superior heat exchanger devices. Heat pipes had been widely used in HVAC applications in energy conservation, dehumidification enhancement, heat dissipation, etc. A number of researches have been conducted to expand the applicability of heat pipes in HVAC in Malaysia, especially in air-to-air heat recovery using stationary heat pipes. However, the potential usage of rotating heat pipe in heat recovery in tropical countries like Malaysia was yet to be explored. Hence, the potential of rotating heat pipe in the HVAC systems used in tropics was explored through a parametric study that incorporates rotational speeds, off-axis displacements and varied refrigerants. The rotating heat pipes charged with R134a, R22 and R410A were tested with varied radial displacement from the rotational axis. The straight and leveled heat pipe with the furthest radial displacement yields the most significant heat transfer, which was attributed to the magnitude of the generated centrifugal force, and effective distribution of liquid in the evaporator.