Correlation For Heat Transfer Coefficient Research Articles

Frosting characteristics of microchannel heat exchangers: Parametric studies and correlation development

Recently, microchannel heat exchangers (MCHX) have been widely used in heat pump systems due to their high surface-to-volume ratio and high efficiency of heat transfer. However, when the MCHX operates in frosting conditions, its performance is adversely affected. To study the frost formation and distribution characteristics of MCHX, a comprehensive parametric study was performed for the MCHX with a larger fin pitch. The effects of fin geometries and environmental parameters on the frosting characteristics of MCHX were studied. Fin height (Fh) can effectively inhibit frost formation, which is due to the increase of surface temperature with the increase of Fh. When Fh increased from 6.0 mm to 11.8 mm, the frost time increased by 30.5 %. However, the increase in Fh also brings the penalty of reduced heat transfer rate. The effect of fin depth (Fd) on frost formation performance is small because the frost is mainly deposited in the front part, resulting in the rear fin cannot work efficiently. Fin pitch (Fp) is the most significant structural parameter affecting frosting performance. When Fp increases from 1.6 mm to 4 mm, the frosting time increases by 4 times and frost mass increased by 2.2 times. This is because with the increase of Fp, the frost blocking in the front part can be effectively delayed, and the distribution uniformity of the frost layer is improved. Lower surface temperature and higher humidity ratio significantly promote frost formation and reduce the uniformity of frost distribution. When the inlet temperature is reduced from −5 °C to −8 °C, the frosting time is shortened by 2.15 times, and the frost mass is reduced by 55.7 %. When the relative humidity increases from 76 % to 92 %, the frosting time is shortened by 2.15 times, and the frost mass is reduced by 42.5 %. The air velocity has a minor impact on the growth rate of frost in the front part. However, as the air velocity increases, the uniformity of frost distribution is significantly improved. When the air velocity increased from 1.1 m s−1 to 2.2 m s−1, the mass of accumulated frost increased by 29.5 %. Furthermore, frost thickness and air-side heat transfer coefficient correlations were developed by testing 12 samples. The developed heat transfer coefficient correlation can accurately capture the changing trend of the j-factor. This study provides a fundamental understanding of the frosting characteristics of MCHX and can effectively guide the louvered fin optimization.

Wind-tunnel experiments on cross-ventilative cooling in a generic isolated building with one heated wall: Impact of opening size

This paper presents wind-tunnel experiments of cross-ventilative cooling in a generic isolated building with an interior heated side wall. Two different sizes of openings are considered: large and small openings. Particle image velocimetry (PIV) is used to determine velocities in the vertical centerplane. Air temperatures in the vertical centerplane are measured using negative temperature coefficient (NTC) sensors. Surface temperatures on the heated wall are measured using an infrared camera. Surface heat fluxes are obtained using heat flux sensors. In both cases the indoor airflow is dominated by the jet through the openings, with higher velocities in the building with large openings. The air temperatures measured with small openings are up to 7.5 % larger than those with large openings. The surface heat fluxes are up to 20 % higher in the building with large openings. The interior convective heat transfer coefficients vary considerably across the heated wall for both opening sizes and can be very different (up to 5 times higher) from those obtained by existing internal convective heat transfer coefficient correlations. The measurement results give insight into the complexity of ventilative cooling and can be used to validate computational fluid dynamics (CFD) simulations of cross-ventilative cooling.

An Evaluation of Correlations for Predicting the Heat Transfer Coefficient during the Condensation of Saturated and Superheated Vapors Inside Channels

Condensation heat transfer is involved in many industrial applications. Therefore, it is important to know the relative accuracy of the available methods for predicting heat transfer. Condensation can occur with saturated as well as superheated vapors. Predictive methods for both conditions were evaluated using a wide range of data. Twelve well-known correlations for the condensation of saturated vapor, including the most recent ones, were compared with data for 51 pure fluids and mixtures from 132 sources in horizontal and vertical channels of many shapes. Channel hydraulic diameters were 0.08–49 mm, the mass flux was 1.1–1400 kg/m2s, and the reduced pressure range was 0.0006–0.949. The fluids included water, CO2, ammonia, hydrocarbons, halocarbon refrigerants, various chemicals, and heat transfer fluids. The best predictive technique was identified. The three most commonly used models for heat transfer during the condensation of superheated vapors were studied. They were first compared with test data using measured saturated condensation and forced convection heat transfer coefficients to select the best model. The selected model was then compared with test data using various correlations for heat transfer coefficients needed in the model. The best correlations to use in the model were identified. The results of this research are presented, as are recommendations for use in design.

A multiphase heat transfer model for spreading wall film coupled with solid wall temperature field

A simplified model for wall film impingement, spreading and evaporation is proposed in this work. The performance of the model was compared with experimental data. For the experimental data, a 2.8 mm diameter diesel drop at 423 K was impinged on a stainless steel wall. The solid surface and embedded temperature was recorded with time. The variation of film spread fraction, which is the ratio of the film spread diameter to the original drop diameter was also recorded with time. The present model deals with solid, liquid and gas phase physics of the wall film. An analytical expression developed previously for the heat flux from the ambient gas into the liquid gas interface, is used to represent the temperature distribution within the film. The correlation for heat transfer coefficient at the solid–liquid interface was developed and the solid temperature distribution is solved using the semi-infinite solid assumption in the 1-D coordinate system. A simplified film spread model was developed as a function of the maximum film spread fraction, which can be derived from energy conservation principles. A vapor sharing algorithm was also developed for film spreading into the neighboring computational cells. The model results were able to capture the initial rise in solid surface temperature during the initial spread of the film. However, the overall steady state temperature was slightly overpredicted. The solid embedded temperature matched well with experimental results. Variation of temperature within the film and the ambient gas temperature of the cell containing the film with time obtained from the model was also shown.

Experimental heat transfer results and flow visualization of horizontal Near-Saturated liquid nitrogen flow boiling in uniformly heated circular tube under Earth gravity

This study investigates 1-ge horizontal flow boiling of liquid nitrogen with a near-saturated inlet along a circular tube of dimensions 8.5-mm inner diameter and 680-mm heated length. Experiments are conducted using a payload developed for eventual parabolic flight experiments. The operating parameters varied are mass velocity of 406.76–1572.77 kg/m2s, inlet quality of −0.05 to −0.01, and inlet pressure of 336.29–493.07 kPa. High speed video recordings are presented to explain two-phase flow structure and regime transitions which are visualized through a transparent tube in a visualization section situated downstream of the heated tube. Recognized flow patterns are bubbly, plug, slug, stratified annular, and annular. Heat transfer results are presented and discussed in terms of flow boiling curve trends, streamwise wall temperature profiles, streamwise heat transfer coefficient (HTC) profiles, and average HTCs. Previous HTC correlations are evaluated against the measured HTC data, of which two are identified for superior accuracy in predicting cryogen data. Finally, this study confirms the reliability and readiness of the payload for subsequent parabolic flight experiments intended for acquisition of microgravity data.

Experimental and Numerical Investigation of Effect of Inclination on sCO2 Heat Transfer in a Circular Pipe

Abstract Supercritical carbon dioxide (sCO2) can be utilized as a working fluid in various thermal systems including large-scale power cycles; portable power production units, centralized coolant systems, and standalone cooling devices. However, the lack of accurate prediction tools such as heat transfer coefficient correlations and insufficient research studies about the mechanisms controlling heat transfer processes are hindering its practical realization for key energy and cooling systems. The overall objective of this study is to extend fundamental knowledge about heat transfer and fluid dynamic processes in conduits pertinent to sCO2 with an emphasis on flow direction and inclination effects. This paper presents the study on effects of gravity, buoyancy on sCO2 flow at temperatures near and away from the pseudo-critical temperature. The experimental setup consists of a high temperature and pressure sCO2 heat transfer loop and flow testing facility. Recently, researched sCO2 heat exchangers can have tubes oriented at different angles such as 45 deg or 90 deg to horizontal. For optimized design of efficient and cost-effective turbomachinery components utilizing sCO2 as the heat transfer fluid, an understanding of convective heat transfer inside a tube/pipe is equally as important as external heat transfer. This paper presents an experimental and numerical study on sCO2 heat transfer at various inclinations with angles ranging from 0 deg (horizontal) to 90 deg (vertical) along with upward and downward flow direction with different inlet temperatures. Thermocouple-based temperature measurement is utilized at multiple locations within the test section axially and circumferentially to study the temperature distributions on the tube surface. Computational fluid dynamics (CFD) simulations have been performed using ANSYS Fluent to complement experimental data. The CFD and experiment have been analyzed against the well-known Gnielinski Nusselt number correlation.

Innovative twist design for solar water heaters: Experimental findings and implications

More effective use of solar energy in hot water systems is made possible by optimising heat transfer and flow resistance. In light of this, Research has been conducted to enhance convective heat transfer while analysing flow friction characteristics for best performance, resulting in the design of novel inserts known as customised twists, which increase the efficiency of the solar hot water system. A conventional, plain tube heater is used as a baseline for the study on thermal characteristics, heat transportation and flow friction attributes. A thorough analysis and comparison of the thermal behaviour of the system with customised twist inserts that have been modified using rods and spacers of different lengths is performed. As a result of the induced disturbance in the fluid particle flow pattern throughout the length of risers, a significant improvement in the coefficient of convective heat transfer is observed. The study validated the correlations for the convective heat transfer coefficient and the resistance coefficient against previous research findings. The findings indicated that the thermal efficiency of the full-length twist-inserted system is almost three times greater than that of a plain collector while doubling its pressure drop. The findings of this research could be useful in designing low-cost, compact solar water heaters.

Assessment and development of saturated and subcooled heat transfer coefficient correlations for cryogenic flow boiling in tubes

The escalating interest in cryogenic technologies for space-related applications has led to an unprecedented demand for reliable prediction methods for cryogenic two-phase flow and heat transfer. Regrettably, existing heat transfer coefficient (HTC) correlations developed for conventional fluids prove inadequate and provide subpar predictions when applied to cryogenic flow boiling conditions. Therefore, it is imperative to develop a set of new HTC correlations specifically tailored for cryogenic flow boiling. In this study, comprehensive databases are constructed consolidating experimental data from previous studies of the present authors and historical data extracted from open literatures, spanning a wide range of operating conditions, flow orientations, and various cryogenic fluids. Subsequently, based on the constructed databases, an assessment of the predictive accuracy of seminal HTC correlations is conducted, followed by the development of new HTC correlations for cryogenic saturated and subcooled flow boiling. The newly developed correlations demonstrate very good predictive accuracy, with an overall mean absolute error (MAE) of 23.84 % and 21.24 % for LN2 saturated and subcooled HTC, respectively, under terrestrial gravity conditions. When assessed against a microgravity dataset, these correlations exhibit equally good predictive accuracy, yielding MAE values of 16.73 % and 25.99 % for LN2 saturated and subcooled HTC, respectively. Furthermore, the universal applicability of the new HTC correlations is ascertained by assessing the correlations across a multitude of cryogenic fluids, including LN2, LHe, LAr, LCH4, and LH2. Impressively, these correlations display outstanding predictive accuracy, with MAE values of 24.01 % and 21.29 % for saturated and subcooled HTC, respectively, underscoring their superior performance across a wide range of cryogenic fluids, validated against 2,445 saturated cryogenic HTC datapoints and 1,553 subcooled cryogenic HTC datapoints. Overall, the new HTC correlations consistently outperform all prior seminal correlations, yielding good predictive accuracy across a diverse spectrum of operating conditions, irrespective of flow orientation and gravity level, for various cryogenic fluids.

New correlation of the subcooled flow boiling heat transfer coefficient for hybrid micro/nanostructured surfaces with high heat flux incidence

Surfaces manipulated with micro/nanostructures have great potential for imparting high heat removal capacity to heat transfer equipment under high heat flux incidence. However, few correlations of heat transfer coefficients have been developed based on high-heat flux experiments of subcooled flow boiling with nano- and microstructured surfaces, which may limit the further application of this technique in industrial engineering. Aiming at recently developed hybrid micro/nanostructured surfaces for subcooled flow boiling performance enhancement, a series of experiments with different surface structure scale specifications, flow conditions, and heat flux incidence ranges were conducted. In total, 530 experimental data points were obtained with heat fluxes ranging from 3.4 to 13.1 MW/m2, pressures from 0.5 to 1.5 MPa, liquid velocities from 1 to 5 m/s, inlet temperatures from 303.15 to 323.15 K, and structured surfaces with wire apertures of 250, 125, and 58 μm. The 28 existing correlations based on smooth surfaces and structured surfaces failed to predict these experimental data. A new enhancement factor-type correlation model, which included four dimensionless numbers that specialized in the effects of the span dimension and depth dimension of the target hybrid micro/nanostructured surfaces, was developed and shown to have good prediction accuracy and a considerable degree of application adaptability, with a mean absolute deviation of 5.7%.

Heat transfer characteristics of passive containment cooling system with equal height difference under steady-state conditions

In the nuclear energy utilization, the passive containment cooling system (PCCS) holds paramount importance, with its heat transfer performance standing as a critical aspect of nuclear safety. This study simulated the flow and heat transfer behaviors of PCCS with equal height difference by APROS software under a broad range of steady state operating conditions. At first, the flow pattern of natural circulation is categorized by phase state of flow through operational parameters. A criterion model of boundary of phase change is proposed with prediction deviation of less than ±25 %. Furthermore, the influence of various operation parameters such as temperature, non-condensable gas content, and pressure on the heat transfer behaviors of natural circulation was also revealed. A new heat transfer coefficient correlation for both single-phase and two-phase conditions is proposed with prediction deviation of ±15 %. Finally, the thermal stratification phenomenon in the containment vessel with non-condensable gas was investigated, indicating that local heat transfer characteristics in the upper region surpass those in the lower section on account of deposition of the non-condensable gas at bottom of the vessel.

Experimental study on enhancement condensation heat transfer in tube by foam metal in presence of non-condensable gas

Condensation heat transfer in tube is widely applied in industrial production, and the heat transfer process is often weakened by non-condensable gas (NCG) in actual production. Enhancing condensation heat transfer is beneficial to improve production efficiency, which has always been a hot topic in current research. Foam metal material with large specific surface area and good thermal conductivity is an ideal material to enhance heat transfer. In order to study enhancement heat transfer effect and optimize structure of foam metal, this paper investigated condensation heat transfer in tube strengthened by foam metal in presence of NCG experimentally. Section shape of foam metal is annular, and the pores per inch (PPI) of foam metals is 10, 15, 20 respectively. The effects of PPI value, steam/air mixture mass flow, and NCG mass fraction on heat transfer coefficient (HTC) and flow resistance are studied. The results reveal the following: (1) Compared with smooth tube, the foam metal enhances heat transfer significantly, and HTC increases by 1.5-2.3 times. (2) At same steam/air mixture mass flow, 10PPI foam metal tube has the highest HTC compared to others. (3) With increase of NCG mass fraction and PPI value, pressure drop increases and the HTC decreases. Based on experimental data, pressure drop and HTC correlations are developed. This paper provides an important technical basis for foam metal material application in enhancement heat transfer area.

Heat transfer in flat tube car radiator with CuO-MgO-TiO2 ternary hybrid nanofluid

An investigation of the performance of a car radiator with CuO-MgO-TiO2 aqueous ternary hybrid nanofluid has been presented to elucidate the effect of nanoparticle concentration, flow rate, and frontal air velocity. The volume concentration of nanoparticles ranges from 0.1% to 0.5%. The effect of fluid inlet temperature, the coolant flow rate and the frontal air velocity on a flat tube radiator were investigated. The effectiveness of the radiator increases with increasing volume concentration as well as coolant flow rate. The coolant side heat transfer coefficient of 0.5% hybrid nano-coolant was 46% higher than the base fluid. The overall heat transfer coefficient was 7.65% higher than the base fluid. The friction factor and the pumping power both decrease with increasing fluid inlet temperature. The pumping power required for nano-coolant is 1.71 times the base fluid. A correlation for the overall heat transfer coefficient is also presented.

Design and optimization of molten salt printed circuit steam generators

Among the array of heat exchangers available, printed circuit steam generators emerge as a particularly promising choice for high-temperature applications. This is primarily attributed to their augmented heat transfer surface area, which enables the efficient transfer of thermal energy within the Rankine power conversion cycle, especially in the context of molten salt-based thermal power systems, such as those employed in molten salt reactors and concentrated solar power plants. It is imperative to analytically model the heat transfer characteristics of mini-channel flow boilings for a printed circuit steam generator. To resolve new technical design issues that the single hot fluid flow channel is cooled by multiple cold fluid flow channels, an improved heat exchanger model is proposed to assist the numerical modeling of printed circuit steam generators for molten salt based on the equilibrium thermodynamics quality of water/steam flow. The existing flow boiling heat transfer coefficient correlations of mini-channels are leveraged to optimally design the straight-passage based printed circuit layouts and narrow down their optimal operational parameters. With various selections of subcooled water, saturated water, and superheated steam heat transfer correlations, The best combination of heat transfer correlations is found to assess the axial distributions of cold fluid temperatures and pressure drops. Two multi-objective optimizations are formulated to respectively solve thermal sizing and scaling problems for the optimal designs and operations of printed circuit steam generators. The computational results indicate that the overall heat transfer coefficient for an optimized 50 MW(th) printed circuit steam generator assessed at the transition point is about 3,828 W/(m2 K), which remarkably surpasses the 1,544 W/(m2 K) value of the conventional shell-tube steam generators by a factor of three. This implies that printed circuit steam generators would be a promising replacement of the traditional shell-tube steam generator.

A correlation of heat transfer for falling film evaporation on the horizontal tube with multiple-precision expansion method

The falling film evaporation of water and seawater outside the horizontal tube is experimentally investigated under non-boiling condition, and the effects of spray density, evaporation temperature, tube pitch, circumferential position and seawater salinity on the falling film evaporation are studied. A competitive mechanism, based on the analysis of liquid film, is proposed to analyze the critical Reynolds number of the spray water, ReΓ, and the dual effects of liquid film on heat transfer coefficient. The experimental data covers tube diameter D=19∼38 mm, tube pitch s/D=1.3∼2.25, evaporation temperature Tsat=50∼70 °C, spray Reynolds number ReΓ=118∼1979, and salinity S=0∼60 g/kg. By summing up a large number of experimental data, the heat transfer coefficient correlation applying both water and seawater falling film evaporation on horizontal tubes is proposed. The correlation is also compared with previous studies about water and seawater, which shows a better and wider adaptability to experimental conditions.

Numerical study of the flow and heat transfer characteristics in a scale model of the vessel cooling system for the HTTR

The reactor cavity cooling system (RCCS) is a passive reactor safety system commonly present in the designs of High-Temperature Gas-cooled Reactors (HTGR) that removes heat from the reactor pressure vessel by means of natural convection and radiation. It is one of the factors responsible for ensuring that the reactor does not melt down under any plausible accident scenario. For the simulation of accident scenarios, which are transient phenomena unfolding over a span of up to several days, intermediate fidelity methods and system codes must be employed to limit the models’ execution time. These models can quantify radiation heat transfer well, but heat transfer caused by natural convection must be quantified with the use of correlations for the heat transfer coefficient. It is difficult to obtain reliable correlations for HTGR RCCS heat transfer coefficients experimentally due to such a system’s size. They could, however, be obtained from high-fidelity steady-state simulations of RCCSs. The Rayleigh number in RCCSs is too high for using a Direct Numerical Simulation (DNS) technique; thus, a Reynolds-Averaged Navier–Stokes (RANS) approach must be employed. There are many RANS models, each performing best under different geometry and fluid flow conditions. To find the most suitable one for simulating an RCCS, the RANS models need to be validated. This work benchmarks various RANS models against three experiments performed on the HTTR RCCS Mockup by the Japanese Atomic Energy Agency (JAEA) in 1993. This facility is a 1/6 scale model of a vessel cooling system (VCS) for the High Temperature Engineering Test Reactor (HTTR), which is operated by JAEA. Multiple RANS models were evaluated on a simplified 2d-axisymmetric geometry. They were found to reproduce the experimental temperature profiles with errors of up to 22% for the lowest temperature benchmark and 15% for the higher temperature benchmarks. The results highlight that the pragmatic turbulence models need to be validated for high Rayleigh natural convection-driven flows and improved accordingly, more publicly available experimental data of RCCS resembling experiments is needed and indicate that a 2d-axisymmetric geometry approximation is likely insufficient to capture all the relevant phenomena in RCCS simulations.

Experimental investigation and correlation development of flow condensation with R123/MWCNTs nanorefrigerant inside a horizontal tube

The current experimental study is focused on examining the behaviour of heat transfer of pure R123 and R123/multi-walled carbon nanotubes nanofluid condensing in a smooth horizontal pipe at a range of different working conditions, like the mass velocity of about 200–500 kg/m2s, multi-walled carbon nanotubes volume fraction of approximately 0.01, 0.05, and 0.1 %, vapour quality of approximately 0.05–0.9, and saturation temperature of about 35.3 and 40.9 °C. The heat transfer coefficient of pure R123 and nanorefrigerant is compared in this investigation. According to the results, employing multi-walled carbon nanotubes improves the heat transfer coefficient for all selected volume fractions. It is observed that the heat transfer coefficient for both pure and nanorefrigerant falls as the saturation temperature rises from 35.3 to 40.9 °C. For a mass velocity of about 400 kg/m2s, 0.1 % fraction of multi-walled carbon nanotubes results in the highest heat transfer coefficient increase of 29 % compared to the base refrigerant. The mean absolute error analysis is evaluated to forecast the accuracy of the experiments by comparing the experimentally captured condensation heat transfer data with available correlations, which shows good agreement. A new correlation for flow condensation heat transfer coefficient is suggested, with an acceptable mean absolute percentage error of about 9.6 %.

A Comprehensive Study on the Effects of Surface Roughness on Power Consumption and Heat Transfer Performance in Novel In-Tube Condensation r123yf Refrigerant Systems

This study investigates the impact of surface roughness on the condensation Heat Transfer Coefficient (HTC) of R123yf refrigerant in a uniquely designed test rig with pre-condenser and post-condenser for quality regulation. In mining industries, the mining equipment consumes more amount to power for the Air Conditioning system. To reduce the power consumption of copper metal inside the surface morphology needs to be changed. Experiments were conducted in a 750 mm long, 8.4 mm internal diameter copper tube, with regard to mass flux varying from 150 kg.m-2.s-1 to 300 kg.m-2.s-1, refrigerant quality varying from 0.3 to 0.8, and saturated temperature from 35°C to 45°C. The impact of varying surface roughness (1.5 μm, 2.5μm, 3.4 μm, and 6.7 μm) on HTC was examined. The plane tube condensation experimental results agreed with M M Shah's heat transfer coefficient correlations, with a 20% absolute mean deviation. It was observed that increasing surface roughness enhanced the HTC. The optimal surface roughness was determined to be 3.4 μm, as it resulted in a significant increase in HTC (26.25%) with a moderate rise in power consumption (15.40%). Higher surface roughness led to a drastic increase in power consumption, making it less desirable for practical applications. Through a systematic analysis of the experimental data, this study identifies the critical trade-off between surface roughness and power consumption, providing guidance for the design of energy-efficient condensation heat transfer systems using R123yf refrigerant. By pinpointing an optimal surface roughness for copper tubes used in refrigeration, this study offers a pragmatic solution for enhancing the efficiency of air conditioning systems in mining equipment, a sector notorious for high energy expenditures. The findings indicate a direct application in mining operations where equipment cooling demands contribute significantly to energy consumption. Implementing the recommended surface modifications can lead to substantial energy savings, thereby reducing operational costs and improving the sustainability of mining activities. Furthermore, the adaptation of R123yf refrigerant systems with optimized surface roughness could serve as a model for energy conservation measures across various facets of the mining industry..

Experimental investigation on condensation heat transfer coefficient and frictional pressure drop of low GWP refrigerant R1234yf on a wavy fin surface

Refrigerant R1234yf is an ideal substitute to replace R134a due to its lower value of global warming potential (GWP). The thermophysical properties of R1234yf are similar with R134a and thus it is considered as a potential replacement for R134a in thermal fluid systems. Further, it was noticed that there is a lack of analysis on the condensation of R1234yf in a brazed plate fin compact heat exchanger (CHE) employing wavy fins. In the present work, condensation heat transfer coefficient and frictional pressure drop of refrigerant R1234yf are investigated in a brazed plate fin compact heat exchanger with wavy fin surfaces. The test condenser with wavy fin surface was selected to examine the condensation heat transfer coefficient and frictional pressure drop using refrigerants R134a and R1234yf for varying mass flux and saturation temperatures. The present study also discusses the dependency of specific kinetic energy of refrigerant on frictional pressure drop. Further, using the experimental data of wavy fin surfaces, suitable correlations for the condensation heat transfer coefficient and frictional pressure drop of R1234yf were proposed. The present paper also reports the comparison made between refrigerants R134a and R1234yf. Comparison between R1234yf and R134a shows that condensation heat transfer coefficient and frictional pressure drop of R1234yf is 9–16 % and 10–17 %, respectively, lower than of R134a in the mass flux range 15–44 kg/m2s.

Experimental study and general correlation for condensation heat transfer coefficient inside microfin tubes

This study investigated the condensation heat transfer characteristics of the pure refrigerants R1234yf and R32 in a microfin tube with a small diameter of 3.5 mm outside diameter and 3.18 mm inner mean diameter. Experiments were carried out at mass velocities between 50 and 200 kg m-2 s-1, saturation temperatures of 20 and 30 °C, and vapor quality between 0 and 1. The effects of vapor quality, mass velocity, saturation temperature, and refrigerant properties on condensation heat transfer characteristics were investigated. A new correlation is also developed to predict condensation heat transfer coefficients that can be applied to horizontal microfin tubes with inner diameters between 2.5 and 8.9 mm. The new correlation is validated with the database containing 1240 experimental data points from 10 independent research groups. The comparison results show that the three existing correlations overestimate the heat transfer coefficient data with mean deviation greater than 30.0 %. Meanwhile, the new correlation predicted the present experimental data and other researchers' data with mean deviation of 19.8 %.

Condensation Performance of Symmetrical and Parallel Capsule Plate Heat Exchangers

Condensation characteristics of symmetrical and parallel capsule plate heat exchangers were experimentally investigated in this article. Plate heat exchangers with parallel capsule corrugation, symmetrical capsule corrugation, and chevron-type corrugation are manufactured as the test sections, each containing 41 plates. Pure steam and water are the hot and cold fluids, respectively. The experiments covered a range of equivalent Reynolds number from 9,000 to 25,000, mass flux of condensing fluid from to 18 to 49 kg/(m2·s), liquid Prandtl number from 1.37 to 1.71, average vapor quality from 0.5 to 0.7, and condensation pressure from 110 to 240 kPa. The correlations of condensation heat transfer coefficient and friction factor of the symmetrical and parallel capsule plate heat exchangers are developed. Compared with chevron-type plate heat exchangers, capsule plate heat exchangers show low friction pressure drop, and 33% higher energy efficiency index for condensation flow and heat transfer. Specifically, the energy efficiency index of the symmetrical capsule plate heat exchanger is 8.6% higher than that of the parallel capsule plate heat exchanger, indicating the superiority of counter-rotating longitudinal vortices over co-rotating longitudinal vortices in condensation, similar to what is observed in single-phase flow and heat transfer.