Heat Transfer Coefficients Of Refrigerants Research Articles

Simulation on the performance and cooling capacity compensation solution of a loop thermosyphon system under fan failure conditions

Fan failure of loop thermosyphon systems (LTS) is a major factor affecting thermal safety of data centers. This paper develops a distributed-parameter model to accurately predict the overall and local heat transfer performance of a loop thermosyphon (the terminal equipment of LTS) under fan failure conditions. The simulation results show that the positions of faulty fans have a significant influence on the evaporator's local airside heat transfer coefficient, but have slight influences on the evaporator's local refrigerant heat transfer coefficient and the condenser's overall heat transfer coefficient, which would not increase the risk of heat transfer performance. Thus, fan airflow rate backup can be used to handle fan failure. Then, the effect of operating parameters including backup airflow rate (Bfan), cold water temperature (Tcw) and increment of cold water flow rate (Im,cw) on the effectiveness and efficiency of the cooling capacity compensation under fan failure conditions is analyzed by using the Taguchi method with an L16 orthogonal array. The Bfan is the primary factor contributing 81.51% on cooling capacity compensation, with Tcw and Im,cw contributing 15.25% and 2.38%, respectively. Considering the energy efficiency, the order of adjustment should be Bfan first, Im,cw second, and Tcw last when fan failure occurs.

Machine Learning Assisted Microchannel Geometric Optimization—A Case Study of Channel Designs

At present, microchannel heat exchangers are widely applied in the fields of air-conditioning and heat pumping applications given their high heat transfer performance, compact size, and low material cost. However, designing and optimizing the channel geometries remain challenging, as they require balancing multiple competing objectives to achieve the optimal performance. This study investigates various parameters, including the channel count, wetted perimeter, cross-sectional area, and mass flow rate for each channel, to achieve the optimal efficiency. The optimization objectives include maximizing the heat transfer rate, minimizing the refrigerant convective thermal resistance, maximizing the refrigerant heat transfer coefficient, and minimizing the pressure drop. A multi-objective genetic optimization algorithm, in conjunction with artificial neural network (ANN)-based machine learning models, was used to predict the heat transfer rate to speed up the calculation process during the optimization. We identified that a gradient reduction in the wetted perimeter from the air inlet along the airflow direction could enhance the heat transfer rate. Additionally, the results indicate that an increase in the number of channels leads to an enhanced heat transfer efficiency rate. However, with the increase in the number of channels, the cross-sectional area of each channel is correspondingly reduced to maintain a consistent overall cross-sectional area. This reduction increases the fluid resistance, leading to an increased pressure drop across the system. This observation is critical for a microchannel design optimization, highlighting the importance of attaining a balance between achieving a higher heat transfer efficiency and maintaining a favorable fluid dynamic performance.

Theoretical and experimental verification for determining pool boiling heat transfer coefficient using fuzzy logic

Finding pool boiling heat transfer coefficient of refrigerant is considered one of the great challenges which affects the refrigeration system in terms of design and power consumed. Yet most of the calculation is done based on empirical equations. The major defect with those equations is that they rely on certain operating range for the pressure and heat flux. Outside that range the correctness of the equation will be weak. A fuzzy logic model to predict the heat transfer coefficient of the pool boiling process is exposed in this research. The model is utilized to find the heat transfer coefficient for refrigerant R-134a at different operating conditions. The operating pressure, the applied heat flux and the surface roughness of brass heating plate are changed to exam the ability of the model to find the heat transfer coefficient correctly. Mamdani method is used to model the fuzzy system. An experimental confirmation is done to authenticate the proposed fuzzy model. The obtained fuzzy logic results have a deviation of ± 2% at low heat flux and maximum deviation is ± 6% at high heat flux. In addition, the attained results from both fuzzy logic and experimental work are compared with previous work and found to have the same qualitative behave and near quantitative with previous results of other researchers worked in the same range.

Investigation of boiling heat transfer coefficients of different refrigerants for low fin, Turbo-B and Thermoexcel-E enhanced tubes using computational smart schemes

The design and manufacture of highly efficient evaporators and heat exchangers in cooling machinery need an accurate estimation of the boiling heat transfer coefficient of the refrigerants. In the present study, the boiling heat transfer coefficients of different refrigerants were predicted using machine learning methods and were compared to the existing empirical correlations. For this purpose, four models of ANN, ANFIS, ELM and SVM were developed by using MATLAB functions. 320 data were collected on the boiling heat transfer coefficient of the refrigerants in the plain and enhanced (low fin, Turbo-B, Thermo excel-E) tubes. The percent deviation from the actual value was between − 2.05 and 1.36% for the ANN, − 4.97 and 8.72% for the ANFIS, − 38.11 and 83.01% for the ELM and − 17.35 and 78.37% for the SVM. The results show that the proposed ANN and ANFIS models are reliable models for predicting the boiling heat transfer coefficient. They have a better performance than the ELM and SVM models. The values of RMSE, AARD and R2 for the best model were 74 W m−2 K−1, 0.399% and 0.99993 for the ANN, 306 W m−2 K−1, 1.117% and 0.99883 for the ANFIS, 2163 W m−2 K−1, 15.539% and 0.94191 for the ELM and 2212 W m−2 K−1, 14.905% and 0.93921 for the SVM. The intelligent algorithms of ANN and ANFIS have more accurate predictions than empirical correlations.

Experimental study on heat transfer resistance of condensation heat transfer in high efficiency heat transfer

The condensation heat transfer experiment of R134a inside the horizontal enhanced tube was operated under the condensation temperatures of 35°C, 40°C and 45°C. Firstly, correlations were used to predict the heat transfer coefficient inside the tube to check the experimental data reliability. Then mass flux and condensation temperature were selected as the variables, and the total heat transfer coefficient K, the water heat transfer coefficient hw and the refrigerant heat transfer coefficient hr were seen as performance evaluation index to study the heat resistance of the enhanced tube. Experimental results show that: both the total heat transfer coefficient K and the refrigerant heat transfer coefficient hr increase with increasing mass flux, and get bigger with temperature decreasing. But the water heat transfer coefficient hw slightly decreases with mass flux increasing. And condensation temperature has a small influence on it. Analyze the thermal resistance and find that the water thermal resistance increases and the refrigerant thermal resistance decreases with mass flux increasing but the refrigerant thermal resistance is always less than the water thermal resistance. The difference between them gets bigger with the temperature decreasing.

Effect of SiO2 nanoparticles on the heat transfer characteristics of refrigerant and tribological behaviour of lubricant

Scientists and researchers are enthusiastically utilizing novel technologies that can substantially reduce energy consumption in diverse thermal systems. The foremost aim of the present investigation is to exploit the potential of nanoparticles on the enhancement of heat transfer characteristics of refrigerant and tribological capabilities of lubricant. Experimental investigations on the forced convection heat transfer characteristics of R134a/SiO2 nanorefrigerant (R134a + SiO2-polyalkylene glycol nanolubricant) are carried out. The effect of particle concentration, heat flux, mass flux, and vapour quality are investigated. The friction reduction and anti-wear properties of the nanolubricant are evaluated using four-ball tribo-tester. The results reveal that the presence of nanoparticles in the refrigerant significantly enhances the heat transfer coefficient of refrigerant and the tribological behaviour of lubricant. The maximum increase in heat transfer coefficient is 163.2% corresponding to a particle concentration of 0.4%. The presence of nanoparticles in refrigerant results in a marginal increase in pressure drop. However, the penalty in pressure drop is insignificant compared to the enhancement in heat transfer. Excellent friction reduction and anti-wear performance are also manifested by the nanolubricant. The maximum reduction in the friction coefficient is 37.76% and that of wear scar diameter is 40.83%. Considering the heat transfer and tribological characteristics, the optimum value of the particle volume fraction is 0.4%. SiO2-PAG nanolubricant can be considered as a potential energy-saving alternative to refrigerant compressor oils.

Experimental study of enhanced PCM exchangers applied in a thermal energy storage system for personal cooling

The personal cooling device investigated is based on the vapor compression cycle and has a condenser integrated with the phase change material (PCM), in order to increase building thermostat setpoint and provide high-level human comfort. Since the PCM thermal conductivity is low, the objective of this study is to investigate five PCM exchanger designs with different heat transfer enhancement methods including increasing heat transfer area, embedding conductive structures, and using uniform refrigerant distribution among condenser branches. In the proposed system, PCM exchanger capacity was approximately constant, which is different from most of the previous works. Various refrigerant heat transfer coefficient along condenser length, refrigerant flow maldistribution and PCM natural convection could result in the uneven PCM melting, leading to higher condensing temperature. The graphite matrix exchanger with uniform refrigerant distribution performs the best with 5.5 times higher overall heat transfer coefficient and 49% increased coefficient of performance.

Comparison of Heat Transfer Coefficient of R-12, R-134a and R-409a for Condensation Based on Existing Correlations

The heat transfer coefficient of refrigerants R-12 (dichlorodifluoromethane), R-134a (1, 1, 1, 2, tetrafluoroethane) and R-409a (60 % R22, 25 % R124 and 15 % R142b) are compared on the basis of six existing correlations at condensing temperature 45 °C. Heat transfer coefficients are measured for horizontal tube having internal diameter 8 mm, external diameter 9.52 mm and length of 5 m, mass flux varies from 25 to 450 kg/m2sand quality (dryness fraction) varies from 0 to 1. In the comparison of three refrigerants, Bohdal correlation predict that heat transfer coefficient for R-409a is higher than R-134a and R-12 while according to Cavallini and Zecchin, Shah, Traviss, Huang and Park correlation heat transfer coefficient for R-134a is higher than R-409a and R-12.

Enhancement of vapor compression cycle performance using nanofluids

Recently, Egypt is facing an energy problem due to the increase in consumption and population. There are two ways to face this issue; first, the world should be more interested in renewable energy resources and the second is the efficient use of energy. Refrigeration and air conditioning systems have a high rate of electrical power consumption. For that, the objective of the present work is to enhance the performance of the vapor compression cycle as well as to reduce the energy consumption resultantly. To achieve these goals, the performance of a vapor compression cycle with nanomaterials additives to the primary loop of refrigeration (refrigerant loop) is investigated experimentally. Mineral oil and polyol ester oil with Al2O3 nanomaterials additives are used to enhance the performance in the vapor compression cycle with R-143a refrigerant. The stability of nanofluids was first tested by using sedimentation test. The results showed that the optimum concentration for nanolubricant is 0.1% mass percentage. Results revealed that the refrigerant heat transfer coefficient increased by 22% maximum when nanofluids were used. Moreover, exergy efficiency increases by 20% when mineral oil and Al2O3 nanoparticles were used. The experimental results indicate that R-134a and mineral oil with Al2O3 nanoparticles enhance the vapor compression cycle performance by 22.5% theoretically and 10% actually with 10% less energy consumption. These results were obtained with 0.1% mass fraction of nanolubricant oil. Moreover, experimental results indicate that the polyester oil with Al2O3 nanoparticles mixture has better performance than mineral oil with Al2O3 nanoparticles mixture by 7.5% in theoretical COP and 19.5% in actual COP.

Convective Boiling of R-134a Near the Micro-Macroscale Transition Inside a Vertical Brazed Plate Heat Exchanger

Heat transfer and pressure drop characteristics of R-134a boiling in a chevron-patterned brazed plate heat exchanger (BPHE) are studied experimentally. With corrugated BPHE channels having hydraulic diameter of 3.4 mm and low refrigerant mass flux, boiling near the micro-macroscale transition is speculated. Heat exchanger performance is characterized with varying mass flux (30–50 kgm−2s−1), saturation pressure (675 kPa and 833 kPa), heat flux (0.8 and 2.5 kWm−2), and vapor quality (0.1–0.9). The two-phase refrigerant heat transfer coefficient increases with heat flux as often observed during nucleate boiling. It also weakly increases with saturation pressure and the associated lower latent heat during convective boiling; heat transfer is improved by the decreased liquid film thickness surrounding confined bubbles inside the narrow BPHE channels, which is the main characteristic of microscale boiling. As often observed in macroscale boiling, the inertial forces of the liquid and vapor phases cause an unsteady annular film, leading to premature partial dryout. The onset of dryout is accelerated at the lower saturation pressure, due to increased surface tension, another microscale-like characteristic. Higher surface tension retains liquid in sharp corners of the corrugated channel, leaving lateral surface areas of the wall dry. Two-phase pressure drop increases with mass flux and vapor quality, but with decreasing saturation pressure. Dryout decreases the friction factor due to the much lower viscosity of the gas phase in contact with the wall. Several semi-empirical transition criteria and correlations buttress the current analyses that the thermal-fluidic characteristics peculiar to BPHEs might be due to macro-microscale transition in boiling.

An experimental study and development of new correlation for condensation heat transfer coefficient of refrigerant inside a multiport minichannel with and without fins

This paper is dedicated to the experimental study and development of new correlation for condensation heat transfer in horizontal rectangular multiport minichannel with and without fins using R134a. A new experimental apparatus has been fabricated in order to obtain explicit local condensation heat transfer coefficient measurements over a range of test conditions. The test section is an 852mm long horizontal rectangular multiport minichannel with and without fins having 20 channels with a hydraulic diameter of 0.64mm and 0.81mm. The measurements were done over a range of saturation temperature from 30 to 35°C with mass fluxes ranging from 50 to 200kg/m2s. The effects of vapor quality, mass fluxes, channel geometry, and saturation temperature on the heat transfer coefficient have been clarified and analyzed. The experimental results were compared with the well-known condensation heat transfer models available in the open literature. All of the existing correlations were failed to capture the present experimental heat transfer coefficient with a high degree of accuracy. Asa consequence, a new condensation heat transfer correlation was developed based on the present experimental data and validated with 750 data points collected from the available journal.

Fundamental basis and implementation of shell and tube heat exchanger project design: condenser and evaporator study

A shell and tube heat exchanger is used as a condenser and an evaporator in this theoretical study. Parametric performance analyses for various actual refrigerants were performed using well-known correlations in open sources. Condensation and evaporation were occurred in the shell side while the water was flowing in the tube side of heat exchanger. Heat transfer rate from tube side was kept constant for condenser and evaporator design. Condensing temperatures were varied from 35 to 60 °C whereas evaporating temperatures were ranging from −15 to 10 °C for the refrigerants of R12, R22, R134a, R32, R507A, R404A, R502, R407C, R152A, R410A and R1234ZE. Variation of convective heat transfer coefficients of refrigerants, total heat transfer coefficients with Reynolds numbers and saturation temperatures were given as validation process considering not only fouling resistance and omission of it but also staggered (triangular) and line (square) arrangements. The minimum tube lengths and necessary pumping powers were calculated and given as case studies for the investigated refrigerants considering validation criteria. It was understood that refrigerant type, fouling resistance and arrangement type are one of the crucial issues regarding the determination of heat exchanger’s size and energy consumption. Consequently, R32 and R152a were found to require the shortest tube length and lowest pumping power in the condenser, whereas R507 and R407C have the same advantages in the evaporator. Their heat transfer coefficients were also determined larger than others as expectedly.

Effect of variable heat input on the heat transfer characteristics in an Organic Rankine Cycle system

This paper analyzes the heat transfer characteristics of an ORC evaporator applied on a diesel engine using measured data from experimental work such as flue gas mass flow rate and flue gas temperature. A mathematical model was developed with regard to the preheater, boiler and the superheater zones of a counter flow evaporator. Each of these zones has been subdivided into a number of cells. The hot source of the ORC cycle was modeled. The study involves the variable heat input's dependence on the ORC system's heat transfer characteristics, with especial emphasis on the evaporator. The results show that the refrigerant's heat transfer coefficient has a higher value for a 100% load from the diesel engine, and decreases with the load decrease. Also, on the exhaust gas side, the heat transfer coefficient decreases with the decrease of the load. The refrigerant's heat transfer coefficient increased normally with the evaporator's tube length in the preheater zone, and then increases rapidly in the boiler zone, followed by a decrease in the superheater zone. The exhaust gases’ heat transfer coefficient increased with the evaporator’ tube length in all zones. The results were compared with result by other authors and were found to be in agreement.

Investigation of two-phase heat transfer coefficients of argon–freon cryogenic mixed refrigerants

Mixed refrigerant Joule Thomson refrigerators are widely used in various kinds of cryogenic systems these days. Although heat transfer coefficient estimation for a multi-phase and multi-component fluid in the cryogenic temperature range is necessarily required in the heat exchanger design of mixed refrigerant Joule Thomson refrigerators, it has been rarely discussed so far. In this paper, condensation and evaporation heat transfer coefficients of argon–freon mixed refrigerant are measured in a microchannel heat exchanger. A Printed Circuit Heat Exchanger (PCHE) with 340μm hydraulic diameter has been developed as a compact microchannel heat exchanger and utilized in the experiment. Several two-phase heat transfer coefficient correlations are examined to discuss the experimental measurement results. The result of this paper shows that cryogenic two-phase mixed refrigerant heat transfer coefficients can be estimated by conventional two-phase heat transfer coefficient correlations.

Genetic optimization of heat transfer correlations for evaporator tube flows

Two-phase heat transfer coefficients for internal flows play a critical role in the design and analysis of evaporators and condensers. Previous studies propose empirical relations that combine the effects of nucleate and convective boiling onto the overall heat transfer coefficient. Although these relatively simple empirical relations offer physical insight on the nucleation, boiling and flow processes, they come at the expense of some computational accuracy. In this work, we explored new techniques to determine two-phase heat transfer coefficients for refrigerants R-22, R-134a and R-404a. We used multiple functional forms for the heat transfer coefficients and considered multiple dimensionless parameters as inputs to the algebraic relations. We used genetic algorithms to search the solution space that consists of the input parameters plus the different functional forms, and obtained optimal empirical correlations that cover a wide range of heat transfer regimes. Then, we combined genetic algorithm and artificial neural networks to obtain a more universal correlation. Two versions were developed for each correlation: one that assumes a priori knowledge of the local heat flux and another that does not. Several error metrics were computed for all the correlations developed and compared against correlations from the literature. We conclude that substantial improvements can be achieved in both accuracy and robustness of the correlations by using advanced optimization techniques.

Experimental Assessment of Heat Transfer Correlations for a Small-Scale Evaporative Condenser

This work presents an experimental analysis of a small-scale evaporative condenser, performed on a calorimetric test facility, where heat transfer coefficients are measured and compared to some well-known correlations from the literature. The external flow of air and spray water ranged from 2.2 kg/min to 8.7 kg/min and 4.8 kg/min to 15.0 kg/min, respectively, keeping an average air to water ratio of approximately 1:2, followed by condensation temperatures ranging from 26 to 36°C. The flow pattern map is first determined, followed by the identification of the transition regions based on the void fraction concept. The overall heat transfer coefficient for the condensation zone calculated after the experimental data acquired in the present research was compared to six literature correlations, and the one developed by Tovaras, Bykov, and Gogolin in 1984 provided the better agreement. Local and mean values of the refrigerant heat transfer coefficients did not vary significantly for both single-phase superheated vapor and subcooling liquid. Results are still particular to the evaporative condenser assessed in the present work, and full-scale analysis must be performed in order to build more general correlations.

Influence of metal foam on heat transfer characteristics of refrigerant-oil mixture flow boiling inside circular tubes

The influence of metal foam on heat transfer characteristics of refrigerant and oil mixture flow boiling inside circular tubes was experimentally investigated on two horizontal circular tubes filled with 5 PPI and 10 PPI copper foams, respectively. The experimental conditions include saturation pressure of 995 kPa, mass flux from 10 to 30 kg m−2 s−1, heat flux from 3.1 to 9.3 kW m−2, inlet vapor quality from 0.175 to 0.775 and oil concentration from 0% to 5%. The experimental results reveal that metal foam enhances the flow boiling heat transfer coefficient of refrigerant by a maximum of 185%, while the enhancement effect of metal foam is weakened at oil presence conditions. A new correlation is developed, which can agree with 98% of the experimental data within a deviation of ±30%.

Average Heat Transfer Coefficient for Pool Boiling of R-134a and R-123 on Smooth and Enhanced Tubes (RP-1316)

This paper presents information about nucleate pool boiling of R-134a and R-123 on TBIIHP and TBIILP tubes, respectively, as well as on smooth tubes. This is the first phase of ASHRAE project RP-1316, Experimental Evaluation of Heat Transfer Impacts of Tube Pitch in a Highly Enhanced Surface Tube Bundle. Data were taken at a saturation temperature of 4.44°C (40°F). The test tubes' outer diameter was 19.05 mm (0.75 in.) and had lengths of 1 m (39.37 in.). Tubes were water heated with an insert in the water passage. A new analytical method known as enthalpy-based heat transfer analysis (EBHT) was introduced and used to determine the refrigerant heat transfer coefficient. The heat flux range was 4–135 kW/m2 (1269–42,823 Btu/h·ft2) for R-134a on TBIIHP and 5–60 kW/m2 (1586–19,033 Btu/h·ft2) for R-123 on TBIILP. The resulting heat transfer coefficient range for R-134a on TBIIHP was 6270–23,268 W/m2·°C (1105–4100 Btu/h·ft2·°F) and 6748–23,338 W/m2·°C (1189–4113 Btu/h·ft2·°F) for R-123 on TBIILP. The present study provided one of the widest heat flux ranges studied with this type of tubes and showed significant structure to the pool boiling curve, which had not been traditionally observed. The experimental data of the smooth tubes were compared against the Cooper and Stephan-Abdelsalam correlations. The Cooper correlation provided a better fit with the experimental data, with an average uncertainty of 27% for R-123 and 16% for R-134a.

Influence of carbon nanotubes on nucleate pool boiling heat transfer characteristics of refrigerant–oil mixture

Influence of carbon nanotubes (CNTs) on nucleate pool boiling heat transfer characteristics of refrigerant–oil mixture was investigated experimentally. Four types of CNTs with the outside diameters from 15 nm to 80 nm and the lengths from 1.5 μm to 10 μm were used in the experiments. Test conditions include CNTs mass fractions in the CNTs nanolubricant from 0 to 30 wt% and CNTs nanolubricant mass fractions from 0 to 5 wt%. The experimental results indicate that the presence of CNTs enhances the nucleate pool boiling heat transfer coefficient of R113-oil mixture by a maximum of 61% under the present test conditions, and the enhancement increases with the decrease of CNTs outside diameter or the increase of CNTs length. For fixed CNTs physical dimension, the enhancement increases with the increase of CNTs mass fraction in the CNTs nanolubricant or the decrease of CNTs nanolubricant mass fraction. A correlation for predicting the nucleate pool boiling heat transfer coefficient of refrigerant–oil mixture with CNTs is proposed, and it agrees with 96% of the experimental data within a deviation of ±10%.

Condensation heat transfer and pressure drop of R134a in annular helicoidal pipe at different orientations

Experimental investigations were conducted to determine the condensation heat transfer and pressure drop of refrigerant R134a in annular helicoidal pipe at three inclination angles. The experiments were performed with the Reynolds number of R134a ranging from 60 to 200, and that of cooling water from 3600 to 22 000; temperatures of R134a at 30 °C and 35 °C, and cooling water at 16 °C, 20 °C and 24 °C. The experimental results indicated that the refrigerant Nusselt number was larger at lower refrigerant saturation temperature, and would increase with the increase of mass flow rates of refrigerant and cooling water. It was found that the refrigerant heat transfer coefficient of annular helicoidal pipe could be two times larger than that of equivalent plain straight pipe when the refrigerant Reynolds number was larger than 140. Comparison with identical helicoidal pipe with opposite flow channel arrangement revealed that the refrigerant heat transfer rate was larger when the refrigerant was flowing in the annular section at the cooling water Reynolds number larger than 4000, but the pressure drop was always larger in this flow channel arrangement.