Evaporation Heat Transfer Coefficient Research Articles

내경 4.57mm 관내 CO2의 증발 열전달 특성

The evaporation heat transfer coefficient of (R-744) in a horizontal tube was investigated experimentally. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and evaporator (test section). The test section consists of a smooth, horizontal stainless steel tube of inner diameter of 4.57mm. The experiments were conducted at mass flux of 400 to , saturation temperature of 5 to , and heat flux of 10 to . The test results showed the heat transfer of has a greater effect on nucleate boiling more than convective boiling. Mass flux of does not affect nucleate boiling too much. In comparison with test results and existing correlations, All of the existing correlations for the heat transfer coefficient underestimated the experimental data. However Jung et al.`s correlation showed a good agreement with the experimental data. Therefore, it is necessary to develope accurate predictions determining the evaporation heat transfer coefficient of in horizontal tubes.

Transient analysis of boiling heat transfer in periodic drying out miniature pools

The evolution of the two-phase thermal control technique is moving in the direction of the use of devices which operate in stable periodic or chaotic unsteady regimes. In these devices the heat transfer coefficient relative to the evaporator walls therefore changes over time and it is hardly predictable, especially in the case of boiling regime. This paper deals with the analysis of the boiling coefficient evolutions over time during the operations of a thermal control devices which periodically operates, the Periodic Two-Phase Thermosyphon (PTPT). An experimental technique for measuring the transient boiling heat transfer coefficient in a thick flat evaporator is shown. This technique has been used to measure boiling heat transfer coefficients over time in the case of evaporators with small pools of liquid (lower than 64 ml) which periodically dries out. The FC72 fluid has been used in the experiments. The results show that no nucleate boiling regime has been observed in the evaporator, which works in an unstable transitional boiling regime with relative heat transfer coefficients lower than those typical of nucleate boiling regimes (lower than 50%).

Experimental Study of Interfacial Shear Stress for an Analogy Model of Evaporative Heat Transfer

In this study, we conducted measurements of an evaporative interfacial shear stress in a passive containment cooling system (PCCS). An interfacial shear stress for a counter-current flow was measured from a momentum balance equation and the interfacial friction factor for evaporation was evaluated by using experimental data. A model for the evaporative heat transfer coefficient of a vertical evaporative flat surface was developed based on an analogy between heat and momentum transfer. It was found that the interfacial shear stress increases with the Jacob number, which incorporates the evaporation rate, and the air and water Reynolds numbers. The relationship between the evaporative heat transfer and the interfacial shear stress was evaluated by using the experimental results. This relationship was used to develop a model for an evaporative heat transfer coefficient by using an analogy between heat and mass transfer. The predictions of this model were found to be in good agreement with the experimental data obtained for evaporative heat transfer by Kang and Park.

12.7mm 내면핀관을 이용한 R-1270의 열전달 특성

This paper deals with the heat transfer characteristics of R-290 (Propane), R-600a (Iso-butane) and R-1270 (Propylene) as an environment friendly refrigerant and R-22 as a HCFC's refrigerant for evaporating. The experimental apparatus has been set-up as conventional vapor compression type refrigeration and air-conditioning system. The test section is a horizontal double pipe heat exchanger. Evaporating heat transfer measurements were performed for smooth tube with the outer diameters of 12.70, 9.52 and 6.35 ㎜ and micro-fin tube 12.70 ㎜, respectively.<BR> For the smooth and micro-fin tubes measured in this study, the evaporating heat transfer coefficient was enhanced according to the increase of the mass flux and decrease of the tube diameter. The local evaporating heat transfer coefficients of hydrocarbon refrigerants were superior to those of R-22 and the maximum increasing rate of heat transfer coefficient was found in R-1270. The average evaporating heat transfer coefficients in hydrocarbon refrigerants showed 20 to 28% higher values than those of R-22. Also, the evaporating heat transfer coefficients of R-22 in the tube diameter of the 12.70 ㎜ smooth and micro-fin tube were compared. Generally, the local heat transfer coefficients for both types of tubes increased with an increase of the mass flux. The heat transfer enhancement factor (EF) between smooth and micro-fin tube varied from 1.9 to 2.7 in all experimental conditions.

혼합냉매 R-407C의 증발 열전달과 압력강하

The evaporation heat transfer coefficient and pressure drop of R-22 and R-407C in a horizontal copper tube were investigated experimentally. The main components of the refrigerant loop are a receiver, a compressor, a mass flow meter, a condenser and a double pipe type evaporator (test section). The test section consists of a smooth copper tube of 6.4 mm inner diameter. The refrigerant mass fluxes were varied from 100 to and the saturation temperature of evaporator were . The evaporation heat transfer coefficients of R-22 and R-407C increase with the increase of mass flux and vapor quality. The evaporation heat transfer coefficients of R-22 is about higher than that of R-407C. The evaporation pressure drop of R-22 and R-407C increase with the increase of mass flux. The pressure drop of R-22 is similar to that of R-407C. In comparison with test results and existing correlations, correlations failed to predict the evaporation heat transfer coefficient of R-22 and R-407C. therefore, it is necessary to develope reliable and accurate predictions determining the evaporation heat transfer coefficient of R-22 and R-407C in a horizontal tube.

수평관내 R-290과 R-600a의 증발 열전달 특성

본 논문은 수평 이중관식 열교환기 내 프레온계 냉매의 대체냉매인 탄화수소계 냉매(R-290과 R-600a)의 증발 열전달 계수에 대한 실험적 결과를 나타내었다. 증발기의 내관 내경은 각각 10.07 mm와 6.54 mm의 2가지를 사용하였다. 증발실험은 질량유속 <TEX>$35.5{\sim}210.4\;kg/m^2s$</TEX>이고, 냉각용량 <TEX>$0.95{\sim}10.1\;kW$</TEX>인 조건에서 수행하였다. 실험결과를 요약하면, 탄화수소계 냉매 R-290과 R-600a의 증발 열전달 계수가 프레온계 냉매 R-22보다 높았다. 즉, R-22의 증발 열전달 계수에 비해, R-290와 R600a는 각각 <TEX>$56.7{\sim}70.1%$</TEX>와 <TEX>$46.9{\sim}59.7%$</TEX> 정도 높았다. 실험데이터와 종래의 상관식을 비교한 결과, 모든 냉매와 관경에 대해서 Kandlikar상관식이 가장 좋은 일치를 보였다. This paper presents the experimental results of evaporation heat transfer coefficients of HC(e.g. R290 and R600a), R-22 as a HCFCs refrigerant in horizontal double pipe heat exchangers, having four different inner diameters of 10.07 mm and 6.54 mm respectively. The experiments of the evaporation process were conducted at mass flux of <TEX>$35.5{\sim}210.4\;kg/m^2s$</TEX> and cooling capacity of <TEX>$0.95{\sim}10.1\;kW$</TEX>. The main results were summarized as follows : The average evaporation heat transfer coefficient of R-290 and R-600a was higher value than that of R-22. In comparison with R-22, the evaporation heat transfer coefficient of R-290 and 600a is approximately <TEX>$56.7{\sim}70.1$</TEX> and <TEX>$46.6{\sim}59.7%$</TEX> higher, respectively. In comparison with experimental data and some correlations, the evaporation heat transfer coefficients are well matched with the Kandlikar's correlation regardless of a type of refrigerants and tube diameters.

Thermal Resistance Measurement across a Wick Structure Using a Novel Thermosyphon Test Chamber

A novel system is developed for measuring the thermal resistance across thin layers of sintered copper wicks of varying porosity. Wicks to be tested are integrated into a passive vertical thermosyphon system, and the resistance is measured for a series of input power levels. The wicks are sintered to a thermally conducting pedestal above a pool of deionized water and heated from below. The apparent thermal resistance across the wick (from the pedestal/wick interface to the vapor space) under the evaporative operating conditions encountered in heat pipes is measured using thermocouples. The apparent thermal resistance across the wick is measured to be as low as 0.01°C/W, corresponding to an evaporative heat transfer coefficient of greater than 128,000 W/m2K.

Experimental investigation of capillary-assisted evaporation on the outside surface of horizontal tubes

Capillary-assisted evaporation is a typical heat transfer method in heat pipes which is characterized by high evaporation coefficient due to extremely thin liquid film. This paper introduces such a micro-scale heat transfer method into normal-scale applications. A series of enhanced heat transfer tubes with circumferential rectangular micro-grooves on the outside surfaces have been experimentally investigated. The aim is to investigate the influence of the tubes’ geometries and operating parameters on the evaporation heat transfer coefficients. In the experiment, the tested tubes are hold horizontally and the bottom surfaces are immersed into a pool of liquid. The heat is added to the thin liquid film inside the micro-grooves through the heating fluid flowing inside the tubes. The factors influencing the capillary-assisted evaporation performance, such as the immersion depth, evaporation pressure, superheating degree, etc. are considered. The experimental results have indicated that there is a positive correlation between the evaporation heat transfer coefficient and evaporation pressure, and negative for the superheating and immersion depth. For water, under the evaporation saturated temperature of 5.0±0.1°C, the superheating of 4.0±0.1°C and the dimensionless liquid level of 1/2, the film side evaporation heat transfer coefficients are 3100–3500W/m2K, which are equivalent to those of the falling film evaporator in LiBr–water absorption machine (2800–4500W/m2K [Y.Q. Dai, Y.Q. Zheng, LiBr–water Absorption Machine, first ed., Chinese National Defence Industry Press, Beijing, China, 1980.]).

Heat transfer characteristics for evaporation of R417A flowing inside horizontal smooth and internally grooved tubes

The experimental study on evaporation heat transfer of R417A (R125/R134a/R600) flowing inside horizontal smooth and two internally grooved tubes with different geometrical parameters was conducted with the mass flow rate range from 176 to 344 kg m −2 s −1, heat flux from 11 to 32 kW m −2, evaporation temperature from 0 to 5.5 °C and vapor quality from 0.2 to 1. Based on the experimental results, the mechanism and role of the mass flow rate, heat flux, vapor quality and enhanced surface influencing the evaporation heat transfer coefficients were analyzed and discussed. In comparison to R22, the evaporation heat transfer coefficients for R417A were lower and much lower in the internally grooved tubes than in the smooth tube. The present experimental results are also compared with the existing correlations, and the modified Kattan model is found to be in much better agreement with the experimental results than the Kattan model. The Koyama and Wellsandt microfin models all tend to over predict the evaporation heat transfer coefficients rather strongly for R417A inside internally grooved tubes.

Experimental Study of Interfacial Shear Stress for an Analogy Model of Evaporative Heat Transfer

In this study, we conducted measurements of an evaporative interfacial shear stress in a passive containment cooling system (PCCS). An interfacial shear stress for a counter-current flow was measured from a momentum balance equation and the interfacial friction factor for evaporation was evaluated by using experimental data. A model for the evaporative heat transfer coefficient of a vertical evaporative flat surface was developed based on an analogy between heat and momentum transfer. It was found that the interfacial shear stress increases with the Jacob number, which incorporates the evaporation rate, and the air and water Reynolds numbers. The relationship between the evaporative heat transfer and the interfacial shear stress was evaluated by using the experimental results. This relationship was used to develop a model for an evaporative heat transfer coefficient by using an analogy between heat and mass transfer. The predictions of this model were found to be in good agreement with the experimental data obtained for evaporative heat transfer by Kang and Park.

A reproducible means of assessing the metabolic heat status of preterm neonates

The aim of the present study was to validate the measurement of metabolic heat production using partitional calorimetry (PC) in preterm neonates exposed to a near-thermoneutral environment in an incubator. In order to reduce experimental uncertainty (due to the different variables involved in the calculation of body heat exchanges between the infant and the environment), the mean radiant temperature and the heat transfer coefficients for convection, radiation and evaporation were measured using a multisegment, anthropometric thermal mannequin which represents a small-for-gestational-age neonate (body surface area: 0.150 m2; simulated birth weight: 1500 g). The metabolic heat production calculated by PC was compared with the results of indirect respiratory calorimetry, which is rarely done in clinical setting since this method interferes with the neonate's environment and requires a high degree of technical preparedness. The oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured in 20 preterm neonates exposed to thermoneutral (32.3 degrees C) and to slightly cool environments (30.2 degrees C). The mean skin temperature was measured by infrared thermography. The measurements were made during well-established periods of active and quiet sleep. Metabolic heat production was assessed by weighting each value of VO2 and VCO2 by the duration of the sleep stages. Our results showed that there was no significant difference between the two methods in terms of their estimation of metabolic activity at thermoneutrality (mean overall difference: 0.34 kJ h(-1) kg(-1)) and in the cool environment (0.26 kJ h(-1) kg(-1)). We observed significant interneonate variability. Partitional calorimetry enabled the prediction of body growth with a daily error of less than 5.3 g (2.38 kJ h(-1) kg(-1)) for all the neonates at thermoneutrality and for 85% of the subjects (3.03 kJ h(-1) kg(-1)) in the cool environment. Despite this limitation, we demonstrate here that PC provides reliable information for calculating the energy expenditure of individual preterm neonates on the basis of standard environmental input variables. We suggest that the technique can be advantageously used to assess the energy expenditure and normal growth of these infants.

Experimental study on the evaporative heat transfer and pressure drop of CO 2 flowing upward in vertical smooth and micro-fin tubes with the diameter of 5 mm

Because of the ozone layer depletion and global warming, new alternative refrigerants are being developed. In this study, evaporation heat transfer characteristic and pressure drop of carbon dioxide flowing upward in vertical smooth and micro-fin tubes were investigated by experiment with regard to evaporating temperature, mass flux and heat flux. The vertical smooth and micro-fin tubes with outer diameter (OD) of 5 mm and length of 1.44 m were selected as a test section to measure the evaporative heat transfer coefficient. The tests were conducted at mass fluxes from 212 to 530 kg/(m 2 s), saturation temperatures from −5 to 20 °C and heat fluxes from 15 to 45 kW/m 2, where the test section was heated by a direct heating method. The differences of heat transfer characteristics between the smooth and the micro-fin tubes were analyzed with respect to enhancement factor (EF) and penalty factor (PF). Average evaporation heat transfer coefficients for the micro-fin tube were approximately 111–207% higher than those for the smooth tube at the same test conditions, and PF was increased from 106 to 123%.

Experimental study of heat transfer performance in a flattened AGHP

Round mini-axial grooved heat pipes (AGHP) with a diameter of 6 mm and a length of 210 mm were pressed into flattened heat pipes with a thickness of 3.5 mm, 3 mm, 2.5 mm and 2 mm, respectively. The article measured the heat transfer limit, thermal resistance and evaporation heat transfer coefficient of the said AGHPs and analyzed and studied the result. The result indicates: the heat transfer limit decreased with the increase of flattening degree. The heat transfer limit of the 2 mm thick flattened AGHP was only 1/4 of that of the φ 6 mm round AGHP. The thermal resistance of the 3.5–2.5 mm thick AGHPs basically maintained stable at around 0.08 °C/W, while the thermal resistance of the 2 mm thick flattened AGHP increased obviously. The variation of the heat transfer coefficient in evaporator section with the change of flattening degree follows a similar rule to the variation of thermal resistance. This article serves as a reference to understanding the heat transfer performance of mini AGHP and to electronic cooling design of AGHP.

Experimental Study on R-410a Evaporation Heat Transfer Characteristics in Oblong Shell and Plate Heat Exchanger

The evaporation heat transfer experiments were conducted with an oblong shell and plate heat exchanger without oil in the refrigerant loop using R-410A, a mixture of 50 wt% R-32 and 50 wt% R-125 that exhibits azeotropic behavior. An experimental refrigerant loop has been established to measure the evaporation heat transfer coefficient h r of R-410A in a vertical oblong shell and plate heat exchanger. Four vertical counter-flow channels were formed in the oblong shell and plate heat exchanger by four plates having a corrugated trapezoid shape of a 45° chevron angle. The upflow of the boiling R-410A in one channel receives heat from the hot downflow of water in the other channel. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature, and vapor quality of R-410A on the measured data were explored in detail. The results indicate that a rise in the refrigerant mass flux causes an increase in the h r . Raising the imposed wall heat flux was found to slightly improve h r . Finally, at a higher refrigerant saturation temperature, the h r is found to be lower. Based on the present data, an empirical correlation of the evaporation heat transfer coefficient was proposed.

Study on the evaporation Heat Transfer Characteristics of R-134a in Small Diameter Tubes

Large diameter tubes have been used until comparatively lately. However, small diameter tubes are largely used because of their high efficiency in heat transfer and low cost, recently. This study focuses on the experimental research of the heat transfer coefficients during evaporation process of R-22 and R-134a in small diameter tubes. The evaporation heat transfer coefficients were measured in smooth horizontal copper tubes with ID 1.77, 3.36 and 5.35 mm. The evaporation heat transfer coefficients in the small diameter tubes (ID ＜7 ㎜) were observed to be strongly affected by the size of tube diameters and to differ from those of general predictions in the large diameter tubes. The heat transfer coefficients of ID 1.77 ㎜ copper tube were higher by 20 and 30 % than those of ID 3.36 ㎜, ID 5.35 mm copper tubes respectively. Also, it was found that it was very difficult to apply some well-known previous predictions (Shah's, Jung’s, Kandlikar's and Oh-Katsuda’s correlation) to small diameter tubes. Based on the data, the new correlation is proposed to predict the evaporation heat transfer coefficients of R-22 and R-134a in small diameter tubes.

Modelling of evaporation heat transfer of pure refrigerants and refrigerant mixtures in microfin tubes

A semi-empirical model to predict the evaporation heat transfer of pure refrigerants and refrigerant mixtures flowing inside microfin tubes has been presented. The heat transfer coefficient correlation developed in the new model takes into account the nucleate boiling component and the forced convention component. The model was validated using a set of around 380 experimental data points compiled from published literature. Comparison shows that the model is capable of predicting evaporation heat transfer coefficients for different refrigerants and refrigerant mixtures within 10-25 per cent mean absolute deviation for the data sets.

4.57 mm 세관 열교환기 내 이산화탄소의 증발열전달 특성

The evaporation heat transfer coefficient of CO₂(R-744) in a horizontal tube was investigated experimentally. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and evaporator(test section). The test section consists of a smooth, horizontal stainless steel tube of inner diameter of 4.57 ㎜. The experiments were conducted at mass flux of 200 to 500 ㎏/㎡s, saturation temperature of -5 to 5℃, and heat flux of 10 to 40 ㎾/㎡. The test results showed the heat transfer of CO₂ has a greater effect on nucleate boiling more than convective boiling. Mass flux of CO₂ does not affect nucleate boiling too much. In comparison with test results and existing correlations, the best fit of the present experimental data is obtained with the correlation of Jung et al. But existing correlations failed to predict the evaporation heat transfer coefficient of CO₂, therefore, it is necessary to develope reliable and accurate predictions determining the evaporation heat transfer coefficient of CO₂ in a horizontal tube.

Evaporation Heat Transfer Characteristics of Hydrocarbon Refrigerants R-290 and R-600a in the Horizontal Tubes

This paper presents the experimental results of evaporation heat transfer coefficients of HC refrigerants (e.g. R290 and R600a), R-22 as a HCFCs refrigerant and R-134a as a HFCs refrigerant in horizontal double pipe heat exchangers, having four different inner diameters of 10.07, 7.73, 6.54 and 5.80 ㎜ respectively. The experiments of the evaporation process were conducted at mass flux of 35.5~210.4 ㎏/㎡s and cooling capacity of 0.95~10.1 ㎾. The main results were summarized as follows : The average evaporation heat transfer coefficient of hydrocarbon refrigerants(R-290 and R-600a) was higher than the refrigerants, R-22 and R-134a. In comparison with R-22, the evaporation heat transfer coefficient of R-134a is approximately -11~8.1 % higher, R-290 is 56.7~70.1 % higher and R-600a is 46.9~59.7 % higher, respectively. In comparison with experimental data and some correlations, the evaporation heat transfer coefficients are well predicted with the Kandlikar's correlation regardless of a type of refrigerants and tube diameters.

Experimental studies on the evaporative heat transfer and pressure drop of CO 2 in smooth and micro-fin tubes of the diameters of 5 and 9.52 mm

Carbon dioxide among natural refrigerants has gained a considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In-tube evaporation heat transfer characteristics of carbon dioxide were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux and tube geometry. Heat transfer coefficient data during evaporation process of carbon dioxide were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 and 9.52 mm. The tests were conducted at mass fluxes of from 212 to 656 kg m −2 s −1, saturation temperatures of from 0 to 20 °C and heat fluxes of from 6 to 20 kW m −2. The difference of heat transfer characteristics between smooth and micro-fin tubes and the effect of mass flux, heat flux, and evaporation temperature on enhancement factor (EF) and penalty factor (PF) were presented. Average evaporation heat transfer coefficients for a micro-fin tube were approximately 150–200% for 9.52 mm OD tube and 170–210% for 5 mm OD tube higher than those for the smooth tube at the same test conditions. The effect of pressure drop expressed by measured penalty factor of 1.2–1.35 was smaller than that of heat transfer enhancement.

OS2-22 アンモニア/水を用いた海洋温度差発電システムにおける組成の影響(OS2 自然エネルギー,循環型社会における動力エネルギー技術)

The Uehara cycle utilizes ammonia/water mixture as working fluid. For the efficient ocean thermal energy conversion (OTEC) system, nonazeotropic mixtures were utilized as working fluid. This paper describes the effect of mass fraction of ammonia on the cycle. As a result, the overall heat transfer coefficient of evaporator and condenser were increased with evaporator inlet mass fraction of ammonia. And that increased the turbine power out put and cycle thermal efficiency. Consequently, the influence of the additional water on the system was less effective for our plant in this experimental condition. Since we should improve the system to advantage of the mixture.