Evaporating heat transfer and pressure drop of hydrocarbon refrigerants in 9.52 and 12.70 mm smooth tube

Experimental results of heat transfer characteristic and pressure gradients of hydrocarbon refrigerants R-290, R-600a, R-1270 and HCFC refrigerant R-22 during evaporating inside horizontal double pipe heat exchangers are presented. The test sections have one tube diameter of 12.70 mm with 0.86 mm wall thickness, another tube diameter of 9.52 mm with 0.76 mm wall thickness was used for this study. The local evaporating heat transfer coefficients of hydrocarbon refrigerants were higher than those of R-22. The average evaporating heat transfer coefficient increased as the mass flux increased. It is showed the higher values in hydrocarbon refrigerants than R-22. Comparing the heat transfer coefficient of experimental results with that of other correlations, the obtained results from the experiments had coincided with most of the Kandlikar’s correlation. Hydrocarbon refrigerants have higher pressure drop than R-22 in 12.7 mm and 9.52 mm. This results form the study can be used in the case of designing heat transfer exchangers using hydrocarbons as the refrigerant for the air-conditioning systems.     

板式换热器传热和阻力特性的实验研究

利用搭建的液-液型板式换热器试验平台,根据实验数据运用定性雷诺数法拟合出传热关联式,找出Nu与摩擦因子f之间的通用关系式,为板式换热器的设计计算提供了依据。运用传热量与功率的消耗比来评价板式换热器的性能,找出了影响其性能的主要因素,进一步澄清了单纯依靠提高流速来增加传热性能是不经济的。     

Heat transfer and pressure drop characteristics of peripheral-finned tube heat exchangers

The peripheral-finned tube is a new geometry aimed at avoiding moisture-condensate blockage hindering of the air-side heat transfer, by allowing for robust air flow pathways. It consists of a porous structure formed by periodic, radial-hexagonal fin arrangements of different radial extents mounted with a 30° offset from its neighboring level. Here, the air-side pressure drop and the heat transfer characteristics of five different heat exchanger prototypes with different geometric characteristics, such as the radial fin length, fin distribution, and heat exchanger length, were evaluated experimentally in an open-loop wind-tunnel calorimeter. The results demonstrate the effective performance, i.e., the pressure drop and heat transfer characteristics, of this new heat exchanger. A one-dimensional theoretical model based on the porous media treatment was also developed to predict the thermal-hydraulic behavior of the heat exchanger. The model incorporates the actual fin geometry into the calculation of the air-side porosity. The air-side permeability is calculated according to the Kozeny–Carman model and the particle-diameter based analysis. The model predicts the experimental data within a few percent RMS, depending on the correlations used for the friction coefficient and interstitial Nusselt number.     

Analysis of heat transfer and pressure drop characteristics in an offset strip fin heat exchanger

A steady-state three-dimensional numerical model was used to study the heat transfer and pressure drop characteristics of an offset strip fin heat exchanger. Water was the heat transfer medium, and the Reynolds number Re_dh ranged from 10 to 3500. Variations in the Fanning friction factor f and the Colburn heat transfer factor j relative to Re_dh were observed. General correlations for the f and j factors were derived, and these could be used to analyze fluid flow and heat transfer characteristics of offset strip fins in the laminar, transition, and turbulent regions. Finally, three performance criteria (j/f, j/f^1/3, and JF) were adopted, and the best performance criteria for the cases Pr = 7 and Pr = 50 were chosen to be JF and j/f^1/3, respectively.     

Numerical study of laminar heat transfer and pressure drop characteristics in a water-cooled minichannel heat sink

With the rapid development of the information technology (IT) industry, the heat flux in integrated circuit (IC) chips cooled by air has almost reached its limit about 100 W/cm². Some applications in high technologies require heat fluxes well beyond such a limitation. Therefore the search of a more efficient cooling technology becomes one of the bottleneck problems of the further development of IT industry. The microchannel flow geometry offers large surface area of heat transfer and a high convective heat transfer coefficient. However, it has been hard to implement because of its very high pressure head required to pump the coolant fluid though the channels. A normal channel could not give high heat flux although the pressure drop is very small. A minichannel can be used in heat sink with a quite high heat flux and a mild pressure loss. A minichannel heat sink with bottom size of 20 mm × 20 mm is analyzed numerically for the single-phase laminar flow of water as coolant through small hydraulic diameters and a constant heat flux boundary condition is assumed. The effects of channel dimensions, channel wall thickness, bottom thickness and inlet velocity on the pressure drop, thermal resistance and the maximum allowable heat flux are presented. The results indicate that a narrow and deep channel with thin bottom thickness and relatively thin channel wall thickness results in improved heat transfer performance with a relatively high but acceptable pressure drop. A nearly-optimized configuration of heat sink is found which can cool a chip with heat flux of 256 W/cm² at the pumping power of 0.205 W. The nearly-optimized configuration is verified by an orthogonal design. The simulated thermal resistance agrees quite well with the result of conventional correlations method with the maximum difference of 12%.     

Condensation pressure drop characteristics of various refrigerants in a horizontal smooth tube

The two-phase pressure drop characteristics of the pure refrigerants R410a, R502, and R507a during condensation inside a horizontal tube-in-tube heat exchanger were investigated to determine the two-phase friction factor, the frictional pressure drop, and the total pressure drop. The two-phase friction factor and frictional pressure drop are predicted by means of an equivalent Reynolds number model. Eckels and Pate's experimental data, presented in Choi et al.'s study provided by NIST, were used in the analysis. In their experimental setup, the horizontal test section was a 3.81 m long countercurrent flow double tube heat exchanger with refrigerant flowing in the inner smooth copper tube (8.01 mm i.d.) and cooling water flowing in the annulus (13.7 mm i.d.). Their test runs were performed at saturated condensing temperatures from 38.33 °C to 51.78 °C while the mass fluxes were between 119 and 617 kg m⁻² s⁻¹ for the horizontal test section. The separated flow model was modified by ten different void fraction models and correlations, as well as six different correlations of friction factors, in order to determine the best combination for the validation of the experimental pressure drop values. Carey's friction factor was found to be the most predictive. The refrigerant side total and frictional pressure drops were determined within ± 30% using the above friction factor and the void fraction combinations of Carey, Baroczy, and Armand for R410a; and those of Carey, Spedding and Spence, and Rigot for R502 and R507a. The equivalent Reynolds number model was modified using the void fraction correlation of Rigot in order to determine the frictional condensation pressure drop and the two-phase friction factor. The effects of vapor quality and mass flux on the pressure drop are discussed in this paper. The importance of using the alternative void fraction and friction factor models and correlations for the separated flow model is also addressed.     

Single-phase pressure drop and heat transfer characteristics of turbulent liquid nitrogen flow in micro-tubes

Experiments have been conducted to investigate the single-phase pressure drop and heat transfer characteristics of liquid nitrogen in four micro-tubes with the diameters of 1.931, 1.042, 0.834 and 0.531 mm. The friction factors are compared with the conventional correlations over a Reynolds number range of 10,000–90,000. The effect of the variable thermal properties of liquid nitrogen, i.e., viscosity and thermal conductivity, on the flow and local heat transfer in the micro-tubes is clarified. The average Nusselt numbers are determined and compared with the correlations for the conventional channels and micro-channels, respectively. It is found that large roughness of the micro-tube causes high friction factor, and the modified Colebrook correlation can well predict the experimental friction factors by using the measured surface roughness. With the increase of liquid nitrogen temperature, the pressure drop decreases as a result of the lower viscosity. Opposite to water, the local heat transfer coefficient of liquid nitrogen flow in the micro-tube drops by 12.5% along the tube. The experimental data show that the average Nusselt numbers for the micro-tubes are higher than those predicted by the correlations for the conventional channels. Taking into account the effect of surface roughness of the micro-tubes on the heat transfer, the modified Gnielinski correlation enables to predict the experimental Nusselt numbers with a mean absolute error (MAE) of 6.4%.     

The heat transfer and pressure drop characteristics of finned tube banks in forced convection (effects of fin height on pressure drop characteristics)

In recent years the requirement for the reduction of energy consumption has been increasing to solve the problems of global warming and the shortage of petroleum resources. For example, in the power generation field, as thermal power generation occupied 60% of the power generation demand, considerable improvement of the thermal efficiency is required. This paper describes the pressure drop characteristics of finned tube banks used for heat exchangers in thermal power generation that were clarified by testing serrated finned tube banks with different fin heights for improved heat transfer and conventional spiral finned tube banks with different fin heights, and an equation to predict pressure drop which is necessary for the heat exchanger design is proposed. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(3): 179–193, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20112     

Experimental Study on Heat Transfer and Pressure Drop Characteristics of Four Types of Plate Fin－and－TUbe Heat Exchanger Surfaces

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加涡产生器的管片式换热器换热与阻力特性的数值研究

通过数值模拟的方法,研究了小翼式涡产生器对错排圆管管片式换热芯子换热与阻力特性的影响,比较了光板与加涡产生器强化板芯的速度场、横向平均Nu数以及平均对流换热系数、阻力系数的变化规律,为进一步提高其换热性能、改进翅片结构、设计新型换热器提供了理论依据。     

Heat transfer and pressure drop penalization terms (exergy losses) during flow boiling of refrigerants

Heat transfer coefficient and pressure drop correlations are used to analyse the boiling heat transfer performance potentials in plain evaporator tubes of several conventional refrigerants and two newer fluorinated propene isomers possessing low global warming potentials. These correlations are used to calculate two penalization quantities expressed in terms of the refrigerant saturation temperature drop due to pressure drop and the driving temperature difference. These penalization terms are combined into a single Performance Evaluation Criterion dubbed Total Temperature Penalization (TTP). Using the two penalization terms and the TTP, several refrigerants, including the newer alternatives R1234yf (CF₃CF=CH₂) and R1234ze(E) (CF₃CH=CHF ) , are evaluated for their boiling heat transfer performance potentials in plain evaporator tubes. Furthermore, the usefulness of the technique is illustrated through several examples of the optimization of evaporator tube length. Copyright © 2012 John Wiley & Sons, Ltd.     

Endwall heat transfer and pressure drop in scale-roughened pin-fin channels

There is a growing requirement for improved heat transfer performance for a number of electronic devices and this dictates a need to further elevate the endwall heat transfer performances for pin-fin channels. Driven by this need, a novel compound heat transfer enhancement (HTE) measure that combines deepened scales and pin-fin array is devised. Characteristics of heat transfer and pressure drop performances in two scale-roughened pin-fin channels with two different pin pitch-to-diameter ratios are compared for both forward and backward flows in the Reynolds Number (Re) range of 1000–30000. Comparisons of heat transfer data, pressure drop measurements and thermal performance factors with previous results collected from a variety of single and compound HTE devices demonstrate the significant augmentations in both heat transfer rates and pressure drop coefficients for the present HTE measure. This present compound HTE measure with scales and pin-fin array demonstrates an enhancement on the heat transfer up to of 22 times of the developed flow references in smooth-walled pipe within the Re range of 1000–30000. Experimental correlations of heat transfer and pressure-drop coefficients for two scale-roughened pin-fin channels with forward and backward flows are derived to assist design applications.     

The heat transfer and pressure drop characteristics of the finned tube banks in forced convection (effects of fin height on heat transfer characteristics)

In recent years the requirement for the reduction of energy consumption has been increasing to solve the problems of global warming and the shortage of petroleum resources. For example, in the power generation field, as thermal power generation occupies 60% of the power generation demand, considerable improvement of thermal efficiency is required. This paper describes the heat transfer characteristics of finned tube banks used for the heat exchanger in thermal power generation that were clarified by testing serrated finned tube banks with different fin heights for improved higher heat transfer and conventional spiral finned tube banks with different fin height. Then an equation to predict the heat transfer coefficient which is necessary for the design of the heat exchanger was proposed. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(3): 194–208, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20111     

Condensation heat transfer and pressure drop characteristics of R-134a in an annular helical pipe

Condensation heat transfer and pressure drop data of R-134a in annular helical pipes is of significant importance to the effective design and reliable operation of helical pipe heat exchangers for refrigeration, air-conditioning, and many other applications. This paper presents the experimental investigation on condensation heat transfer and pressure drop characteristics of R-134a in an annular helical pipe. The average condensation heat transfer coefficients and pressure drops were experimentally determined for R-134a at three different saturated temperatures (35 °C, 40 °C, and 46 °C). The experimental results are compared with the data available in the literature for helical and straight pipes.     

The heat transfer and pressure drop characteristics of finned tube banks in forced convection (comparison of the pressure drop characteristics of spiral fins and serrated fins)

In recent years there has been a growing need for reduction of energy consumption in an effort to solve problems of global warming and the shortage of petroleum resources. For example, in the power generation field, thermal power generation now occupies 60% of the power generation demand, and the need for improved thermal efficiency is thus considerable. In this paper, the pressure drop characteristics of the finned tube banks used for the heat exchanger in thermal power generation were clarified by testing the serrated finned tube banks for improvement of higher heat transfer and the conventional spiral finned tube banks under the same test conditions, and equations for predicting the pressure drop coefficient which is necessary to design the heat exchanger were proposed. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(7): 431–444, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20030     

Numerical investigation of heat transfer and pressure drop in enhanced tubes

This study investigates passive heat transfer enhancement techniques to determine the distribution of temperature and static pressure in test tubes, the friction factor, the heat flux, the temperature difference between the inlet and outlet fluid temperatures, the pressure drop penalty and the numerical convective heat transfer coefficient, and then compares the results to the experimental data of Zdaniuk et al. It predicts the single-phase friction factors for the smooth and enhanced tubes by means of the empirical correlations of Blasius and Zdaniuk et al. This study performed calculations on a smooth tube and two helically finned tubes with different geometric parameters also used in the analyses of Zdaniuk et al. It also performed calculations on two corrugated tubes in the simulation study. In Zdaniuk et al.'s experimental setup, the horizontal test section was a 2.74 m long countercurrent flow double tube heat exchanger with the fluid of water flowing in the inner copper tube (15.57–15.64 mm i.d.) and cooling water flowing in the annulus (31.75 mm i.d.). Their test runs were performed at a temperature around 20 °C for cold water flowing in the annulus while Reynolds numbers ranged from 12,000 to 57,000 for the water flowing in the inner tube. A single-phase numerical model having three-dimensional equations is employed with either constant or temperature dependent properties to study the hydrodynamics and thermal behaviors of the flow. The temperature contours are presented for inlet, outlet and fully developed regions of the tube. The variations of the fluid temperature and static pressure along tube length are shown in the paper. The results obtained from a numerical analysis for the helically tubes were validated by various friction factor correlations, such as those found by Blasius and Zdaniuk et al. Then, numerical results were obtained for the two corrugated tubes as a simulation study. The present study found that the average deviation is less than 5% for the friction factors obtained by the Fluent CFD program while Blasius's correlation has the average deviation of less than 10%. The corrugated tubes have a higher heat transfer coefficient than smooth tubes but a lower coefficient than helically finned tubes. The paper also investigates the pressure drop penalty for the heat transfer enhancement.     

Laminar convective heat transfer and pressure drop in the corrugated channels

The results of the heat transfer characteristics and pressure drop in the corrugated channel under constant heat flux are presented in the present study. The test section is the channel with two opposite corrugated plates which all configuration peaks lie in an in-line arrangement. The corrugated plates with three different corrugated tile angles of 20°, 40°, and 60° are tested with the height of the channel of 12.5 mm. The experiments are done for the heat flux and the Reynolds number in the ranges of 0.5–1.2 kW/m² and 500–1400, respectively. Effect of relevant parameters on the heat transfer characteristics and pressure drop are discussed. Due to the presence of recirculation zones, the corrugated surface has significant effect on the enhancement of heat transfer and pressure drop.     

Exergy transfer effectiveness on heat exchanger for finite pressure drop

In this paper, exergy transfer effectiveness is defined to describe the performance of heat exchangers operating above/below the surrounding temperature with/without finite pressure drop. It is discussed systemically that the effects of heat transfer units number, the ratio of the heat capacity of cold fluids to that of hot fluids and flow patterns on exergy transfer effectiveness of heat exchangers. Furthermore, the results of exergy transfer effectiveness with a finite pressure drop are compared with those without pressure drop when different objective media, such as ideal gas and incompressible liquid, etc. are applied. The detailed comparisons of the exergy transfer effectiveness with heat transfer effectiveness are also performed for the parallel flow, counter flow and cross flow heat exchangers operating above/below the surrounding temperature.     

Pressure drop and heat transfer characteristics of air-rockbed thermal storage systems

Experiments were conducted to determine the pressure drop and heat transfer characteristics of rockbeds with air as the heat transfer medium. Both the pressure drop and the coefficient of volumetric heat transfer between the air and the rockbeds were found to depend upon the rock size and the air flow rate. In addition, the pressure drop also exhibited dependence on the rockbed porosity. The data, however; did not suggest any influence of rockbed porosity and inlet air and initial rockbed temperatures on the volumetric heat transfer coefficient. Relationships are proposed to estimate the pressure drop and volumetric heat transfer coefficients in rockbeds for the range of variables encountered in low temperature storage applications.