Prediction of frictional pressure drop during flow boiling of refrigerants in horizontal tubes: Comparison to an experimental database

An experimental frictional pressure drop database for smooth tubes including 485 points from the literature is compared to four well-known correlations and also to the phenomenological model by Moreno Quibén and Thome. This model is new and has never been compared to those experimental data. The Grönnerud correlation and the flow pattern based two-phase frictional pressure drop model by Moreno Quibén and Thome display the best prediction. Seventy-four percent of the data are predicted by the latter model within a ±30% error band. Furthermore, the model by Moreno Quibén and Thome predicts a maximum pressure drop before the annular-to-dryout transition, i.e. as usually in the annular flow regime. An explicit expression (never proposed before) for the vapor quality corresponding to the maximum pressure drop (x_M) is developed here. Based on this maximum and on the pressure drop for liquid and vapor, a simple linear function is developed for predicting the frictional pressure drop. This method presents the best accuracy and predicts almost 86% of the data within a ±30% error band. This method does not include any new empirical parameters and can be used for a wide range of experimental conditions. Furthermore, the experimental data were also segregated into flow regimes and compared to each individual prediction method. The linear approach presents the best statistics for each flow regime. In addition, an experimental frictional pressure drop database for helically microfinned tubes including 673 points from the literature is also compared to three existing correlations. The Kuo and Wang correlation presents the best statistics, predicting almost 85% of the data within a ±30% error band.     

Flow boiling heat transfer to carbon dioxide: general prediction method

An updated version of the Kattan–Thome–Favrat flow pattern based, flow boiling heat transfer model for horizontal tubes has been developed specifically for CO₂. Because CO₂ has a low critical temperature and hence high evaporating pressures compared to our previous database, it was found necessary to first correct the nucleate pool boiling correlation to better describe CO₂ at high reduced pressures and secondly to include a boiling suppression factor on the nucleate boiling heat transfer coefficient to capture the trends in the flow boiling data. The new method predicts 73% of the CO₂ database (404 data points) to within ±20% and 86% to within ±30% over the vapor quality range of 2–91%. The database covers five tube diameters from 0.79 to 10.06 mm, mass velocities from 85 to 1440 kg m⁻² s⁻¹, heat fluxes from 5 to 36 kW m⁻², saturation temperatures from −25 °C to +25 °C and saturation pressures from 1.7 to 6.4 MPa (reduced pressures up to 0.87).     

Assessment of boiling and condensation heat transfer correlations in the modelling of plate heat exchangers

This paper studies refrigeration cycles in which plate heat exchangers are used as either evaporators or condensers. The performance of the cycle is studied by means of a method introduced in previous papers which consists of assessing the goodness of a calculation method by looking at representative variables such as the evaporation or the condensation temperature depending on the case evaluated. This procedure is also used to compare several heat transfer coefficients in the refrigerant side. As in previous works the models of all the cycle components are considered together with the heat exchanger models in such a way that the system of equations they provide is solved by means of a Newton–Raphson algorithm. Calculated and measured values of the evaporation and the condensation temperatures are also compared. The experimental results correspond to the same air-to-water heat pump studied in other papers and they have been obtained by using refrigerants R-22 and R-290.     

Cryogenic heat transfer research at Oxford: Part 2 — Flow boiling

Part 1 of this paper which appeared in the August 1973 issue of Cryogenics dealt with nucleate pool boiling. This paper describes an experimental research programme to study the flow boiling of helium I with a view to its potential use in cooling systems for superconductive devices.This research includes investigation of flow boiling at both macroscopic and microscopic levels; the macroscopic studies being concerned mainly with saturated convective boiling and the microscopic studies with sub-cooled convective boiling.     

A survey of correlations for heat transfer and pressure drop for evaporation and condensation in plate heat exchangers

Plate Heat Exchangers (PHEs) are used in a wide variety of applications including heating, ventilation, air-conditioning, and refrigeration. PHEs are characterized by compactness, flexible thermal sizing, close approach temperature, and enhanced heat transfer performance. Due to their desirable characteristics, they are increasingly utilized in two-phase flow applications. Detailed research on heat transfer and fluid flow characteristics in these types of exchangers is required to design and use plate heat exchangers in an optimal manner. This paper reviews the available literature on the correlations for heat transfer and pressure drop calculations for two-phase flow in PHEs as an initial process step in order to understand the current research status. Comparative evaluations for some of the existing correlations are presented in the light of their applicability to different refrigerants. Overall, there is a significant gap in the literature regarding two-phase heat transfer and fluid flow characteristics of these types of exchangers.     

Measurement and prediction of heat transfer coefficient on ammonia flow boiling in a microfin plate evaporator

Thermal characteristics of ammonia flow boiling in a microfin plate evaporator are experimentally investigated. Titanium microfin heat transfer surface is manufactured to enhance boiling heat transfer. Longitudinally- and laterally-microfined surfaces are used and those performances are compared. Heat transfer coefficient of microfin plate evaporator is also compared with that of plain-surface plate evaporator. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient are presented and discussed. The experiments are conducted for the range of mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is compared with that predicted by available empirical correlations proposed by other researchers. Modified correlations using Lockhart-Martinelli parameter to predict heat transfer coefficient are developed and they cover more than 87% of the experimental data.     

Assessment of boiling heat transfer correlations in the modelling of fin and tube heat exchangers

A new way to assess the performance of refrigeration system models is presented in this paper, based on the estimation of cycle parameters, such as the evaporation temperature which will determine the validity of the method. This paper is the first of a series which will also study the influence of the heat transfer coefficient models on the estimation of the refrigeration cycle parameters. It focuses on fin and tube evaporators and includes the dehumidification process of humid air. The flow through the heat exchanger is considered to be steady and the refrigerant flow inside the tubes is considered one-dimensional. The evaporator model is discretised in cells where 1D mass, momentum and energy conservation equations are solved by using an iterative procedure called SEWTLE. This procedure is based on decoupling the calculation of the fluid flows from each other assuming that the tube temperature field is known at each fluid iteration. Special attention is paid to the correlations utilised for the evaluation of heat transfer coefficients as well as the friction factor on the air and on the refrigerant side. A comparison between calculated values and measured results is made on the basis of the evaporation temperature. The experimental results used in this work correspond to an air-to-water heat pump and have been obtained by using R-22 and R-290 as refrigerants.     

Finite-element-method calculation of flow and heat transfer in elements of plate exchangers

The finite-element method is used to solve a problem concerning the heat transfer through a wall separating two rectangular channels. Calculations are performed to establish the distributions of temperature, velocity, heat flux, shear stress, and Nusselt number on the walls of the channels.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 5, pp. 747–757, November, 1990.     

Effects of synthetic oil in a compression refrigeration system using R410A. Part II: quality of heat transfer and pressure losses within the heat exchangers

The consequences of the oil rejected by the compressor of a vapour-compression refrigeration system on the operation of the evaporator and condenser are analysed. The modelled prototype uses the mixture of HFC R410A and a synthetic polyolester (POE) oil. The rise of the amount of lubricant circulating in the system leads to a progressive change in the behaviour of the mixture of refrigerant and oil that, for the higher oil mass fraction, evolves like a zeotropic mixture. One also observes that the presence of lubricant is generally associated with a fall of the performances of the heat exchangers, except however in the evaporator where an optimum is observed when the quantity of oil is equal to 0.1% of the total mass of the mixture. Some conclusions are drawn about the choice of correlations for the calculation of the refrigerant side heat transfer coefficient in a plate evaporator.     

Flow and heat transfer characteristics in the channels of plate-type heat exchangers with corrugated plates

Fluid flow in the channels of plate-type heat exchangers is modeled by a two-dimensional flow in plane channels with distributed resistance to fluid flow determined by the nature of the corrugation of the plates. Such an approach permits one to find the distribution of pressure and fluid flow velocity over the plate field, and then, with the use of the semiempirical dependence of heat transfer coefficient on pressure gradient, to determine the distribution and mean value of the heat transfer coefficient on the field of the heat exchanger plate.The paper was submitted to the 2nd Minsk International Heat and Mass Transfer Forum held from the 18th to the 22nd of May, 1992 (see J. Eng. Phys., Vol. 62, No. 6, 1993).Ukrainian Scientific-Research and Design Institute of Chemical Engineering, Khar'kov. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 1, pp. 3–15, January, 1994.     

Post-dryout heat transfer characteristics in horizontal mini-tubes and a prediction method for flow boiling of CO2

The post-dryout heat transfer coefficients were analyzed by differentiating them into two regions, based on their observed trends. One is a transition region. The other is a fully dryout region. The effects of test conditions for heat flux and saturation temperature on the boundaries of the two regions were also investigated, and the existing models for post-dryout heat transfer coefficients were verified for both regions. To develop a prediction method for flow boiling of CO₂, we applied the critical liquid film model, to predict the point of dryout vapor quality. The Yun et al. model and the Groeneveld model are used to estimate the heat transfer coefficients prior to and subsequent to the point of dryout vapor quality, respectively. The former shows a mean deviation of 38%, and the latter gives 36.4% for the three different data sources.     

Two-phase frictional pressure drop in horizontal micro-scale channels: Experimental data analysis and prediction method development

An investigation was conducted on the effects of fluid refrigerant and channel geometry on the frictional pressure drop during two-phase flow inside microchannels. Experimental results for two-phase frictional pressure drop were obtained for the refrigerants R134a, R1234ze(E), R1234yf and R600a in a circular channel and for R134a in square and triangular channels. The experiments were performed for mass velocities from 100 to 1600 kg m⁻² s⁻¹, saturation temperatures of 31 and 41 °C, and vapor qualities from 0.05 to 0.95. The experimental data have been analyzed focusing on the effects of the geometry and fluid on the two-phase pressure drop. Then, the data were compared with the most quoted predictive methods from the literature. Based on the broad database obtained, a new method for prediction of the two-phase frictional pressure drop was proposed. The new method provided accurate predictions of the database, predicting 89% of the results within an error band of ±20%.     

Comparison of experimental pressure drop data for two phase flows to prediction methods using a general model

In this paper, existing and new two phase pressure drop data are used to run an extensive comparison to predictive methods. The database used is for seven refrigerants (R22, R134a, R404A, R407C, R410A, R417A, and R507A) over a wide range of operating conditions. The procedure used for the comparison is a model of general validity since it is independent of the data reduction procedure. Four quoted methods and a new one by Moreno Quibén and Thome are used. The statistical analysis showed that the methods by Grönnerud and by Moreno Quibén and Thome are equally the best. Segregating the data by flow regimes and taking into account for the prediction of the data trends, the method by Moreno Quibén and Thome is able to give reliable predictions in all the range of vapour qualities, especially in the regions of the intermittent flow and dry-out.     

Impact of selected parameters on refrigerant flow boiling heat transfer and pressure drop in minichannels

This paper presents results concerning flow boiling heat transfer in a rectangular minichannel 1 mm deep, 40 mm wide and 360 mm long. The refrigerant flowing in the minichannel, Fluorinert FC-72, was heated by a thin foil microstructured on the side in contact with the fluid. Two types of microstructured surfaces were used: one with evenly distributed microcavities and the other with non-uniformly distributed minicavities. Liquid crystal thermography was applied to determine the temperature of the smooth side of the foil. The paper analyses mainly the impact of the microstructured heating surface and orientation of the minichanel on the heat transfer coefficient and two phase pressure drop. This required calculating the local values of heat transfer coefficient and measuring the pressure drop for different positions of the minichannel with enhanced heating wall. Moreover, the effects of selected thermal and flow parameters (mass flux density and inlet pressure), the geometric parameters, and the type of cooling liquid on the nucleate boiling heat transfer is studied. From the measurement results it is evident that applying a microstructured surface caused an increase in the heat transfer coefficient, which was approximately twice as high as that reported for the smooth surface. The highest values of the coefficient were observed for position 90° (the vertical minichannel) and position 0° (the horizontal minichannel), whereas the lowest were reported for position 180° (the horizontal minichannel). The experimental data concerning the two-phase flow pressure drop was compared with the calculation results obtained by applying nine correlations known from the literature. It is reported that most of the correlations can be used to predict the two-phase flow pressure drop gradient within an acceptable error limit (±30%) only for positions 90° and 135° (the vertical and inclined minichannels, respectively). The lowest agreement between the experimental data and the theoretical predictions was reported for the horizontal positions of the minichannel.     

An experimental investigation of saturated flow boiling heat transfer and pressure drop in square microchannels

In this study, saturated flow boiling characteristics of deionized water in parallel microchannels are investigated experimentally. The silicone microchannel heat sink consists of 29 parallel square microchannels having hydraulic diameters of 150 µm. Experiments have been conducted for four different values of the mass flux consisting of 51, 64.5, 78 and 92.6 kg/m²s and heat flux values from 59.3 to 84.1 kW/m². Inlet temperature of deionized water is kept at 50 ± 1 °C. Heat transfer and pressure drop are examined for varying values of the governing parameters. Simultaneous high-speed video images have been taken as well as temperature and pressure measurements. The flow visualization results lead to key findings for flow boiling instabilities and underlying physical mechanisms of heat transfer in microchannels. Quasi-periodical rewetting and drying, rapid bubble growth and elongation toward both upstream and downstream of the channels and reverse flow are observed in parallel microchannels.     

Evaporation heat transfer and pressure drop of ammonia in a mixed configuration chevron plate heat exchanger

Ammonia is a naturally occurring environment friendly refrigerant with attractive thermo-physical properties. Experimental investigation of heat transfer and pressure drop during steady state evaporation of ammonia in a commercial plate heat exchanger has been carried out for an un-symmetric 30°/60° chevron plate configuration. Experiments were conducted for saturation temperatures ranging from −25 °C to −2 °C. The heat flux was varied between 21 kW m⁻² and 44 kW m⁻². Experimental results show significant effect of saturation temperature, heat flux and exit vapor quality on heat transfer coefficient and pressure drop. Current mixed plate configuration data are compared with previous studies on the same heat exchanger with symmetric plate configurations. This comparison highlighted importance of optimization in selection of the heat exchangers. Correlations for two phase Nusselt number and friction factor for each chevron plate configuration considered are developed. A Nusselt number correlation generalized for a range of chevron angles is also proposed.     

State-of-the-art of two-phase flow and flow boiling heat transfer and pressure drop of CO2 in macro- and micro-channels

A comprehensive review of flow boiling heat transfer and two-phase flow of CO₂ is presented that covers both macro-channel tests (diameters greater than about 3 mm) and micro-channel investigations (diameters less than about 3 mm). The review addresses flow boiling heat transfer experimental studies, macro- and micro-scale heat transfer prediction methods for CO₂ and comparisons of these methods to the experimental database, highlighting the various limitations of current approaches and the divergence of some data sets from others. In addition, two-phase flow pattern results available in the literature are summarized and compared to some of the leading flow pattern maps, showing significant deviations for CO₂ from the maps prepared for other fluids at lower pressures. Available two-phase pressure drop data for CO₂ are also compared to leading prediction methods.     

Intensification of heat transfer and stabilization of the boiling process in the very low pressure region

Various processes of intensifying heat transfer and stabilizing the boiling process for liquids in the very low pressure region are analyzed and studied experimentally. The great effectiveness of using thin hydrophobic screens attached to the surface to be heated was discovered.Translated from Izhenerno-Fizicheskii. Zhurnal, Vol. 20, No. 6, pp. 973–981, June, 1971.