Heat transfer and pressure drop properties of high viscous solutions in plate heat exchangers

Heat transfer and pressure drop characteristics of an absorbent salt solution in a commercial plate heat exchanger serving as a solution sub-cooler in the high loop of triple-effect absorption refrigeration cycle was investigated. The main objectives of this research were to establish the correlation equations to predict the heat transfer and pressure drop and to analyze and optimize the operating parameters for use in the design of absorption systems.In order to conduct above studies, a single-pass cross-corrugated ALFA-LAVAL plate heat exchanger, Model PO1-VG, with capacity of 14,650 W (50,000 Btu/h) was used. In order to evaluate the performance, hot solution inlet temperatures from 55 °C (130 °F) to 77 °C (170 °F), and inlet temperature differences from 14 °C (25 °F) to 20 °C (35 °F) were used. The cold side of the heat exchanger was operated to match the equal heat capacity rate of hot side.Based on the empirical models proposed in the literature, a program was developed and experimental data were curve fitted. From the best-fitted curves, the power-law equations for heat transfer and pressure losses were established and the performance was evaluated.In the hot salt solution side, the Reynolds number was varied from 250 to 1100 and the resulting Nusselt number varied from 7.4 to 15.8. The measured overall heat transfer coefficient U_overall varied from 970 W/m² °C (170 Btu/h ft² °F) to 2270 W/m² °C (400 Btu/h ft² °F) and the Fanning friction factor in the absorbent side of the heat exchanger varied from 5.7 to 7.6. The correlation equations developed to predict the heat transfer and friction factor perfectly agree with the experimental results. Those equations can be used to predict the performance of any solution with Prandtl numbers between 82 and 174, for heat exchangers with similar geometry.     

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.     

Optimal design of plate heat exchangers with and without pressure drop specifications

The traditional design method for plate heat exchangers (PHEs), either ε-number of transfer units (ε-NTU) or logarithmic mean temperature difference method, involves many trials in order to meet the pressure drop constraints. This can be avoided through the developed design method, which takes the full utilization of the allowable pressure drops as a design objective. The proposed method is valid for the design situations with and without pressure drop specifications. In the case of the design with pressure drop specification, only one stream can fully utilize the allowable pressure drop. In the case of no pressure drop specification, allowable pressure drops can be determined through economical optimization. Compared to the previous design method, the proposed method does not require many trial iterations. Instead, all heat exchanger parameters, including plate size, number of passes, path, fluid velocity, etc., are determined in a straightforward way. Moreover, the suggested method can guarantee that the optimized values of allowable pressure drops can be fully utilized simultaneously by the two streams. In addition, the optimal corrugation angle is discussed for the most common chevron-type PHEs.     

Comparative analysis of heat transfer and pressure drop in helically segmented finned tube heat exchangers

Four different semi-empirical models of heat transfer and pressure drop for helically segmented finned tubes in staggered layout were analyzed. The performance of a Helically Segmented Finned Tubes Heat Exchanger on an industrial scale was obtained and the predictions were compared with experimental data. The method used for thermal analysis is the Logarithmic Mean Temperature Difference (LMTD). Comparisons between predictions and experimental data show a precision greater than 95% in heat transfer for a combination between the Kawaguchi and Gnielinski models at a flue gas Reynolds number, based on the outside bare tube, of about 10,000. In the case of pressure drop, there is a precision of approximately 90% for the Weierman model at a Reynolds number, based on the outside bare tube, of about 10,000. And so, the results show that the best flow regime in which heat transfer and pressure drop are optimum, is for a Reynolds number (based on the outside bare tube) of about 10,000.     

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。     

Numerical estimation of pressure drop and heat transfer characteristics in annular‐finned channel heat exchangers with different channel configurations

In this numerical investigation, three‐dimensional analysis has been used to study the effect of finned channels configuration of (circular, square, and triangular shape) and fin spacing with four rows in staggered arrangements. The finite volume method with k‐ ω turbulent model is applied to estimate the heat transfer and flow characteristics. The results illustrate that the development of the boundary layer between the fins surfaces is credited to the finned channels configuration, fin spacing, and Reynolds number. Moreover, the results of pressure drop and heat transfer with various channel configuration and different fin spacings (1.6, 2, and 4 mm) are presented and validated with the available correlations. The triangular‐finned channel with 1.6 mm fin spacing offered higher heat transfer enhancement followed by square‐ and circular‐finned channels. A considerable agreement was observed when the current findings and the existing correlations were compared, with a maximum deviation of 15% for all the cases.     

Turbulent convective transfer in plate heat exchangers

Readily available data on turbulent transfer in plate heat exchangers can be correlated by a heat transfer-energy dissipation analogy:

$\text{Nu}\text{g}{}_1\text{(pr, Vi)}\text{=C}{}_3\text{(fRe}{}_3\text{)}{}_{\mathrm{\delta }}$

in which the Nusselt number modified for changes in the Prandtl number and bulk to wall viscosity ratio Vi is related to the friction factor f and the Reynolds number. The exponent e is a weak function of the coefficient C₃ which depends on the corrugation geometry.When using chevron or herringbone type patterns the heat transfer depends significantly on the angle between the plate corrugation and the main flow direction. If this angle is π/4 the heat transfer per unit of mechanical energy dissipated is a maximum. Although maximum transfer (with maximum pressure drop) is obtained at π/2, a more practical angle giving high transfer at moderate pressure drops in 2π/5.     

Investigation of heat transfer and pressure drop of a counter flow corrugated plate heat exchanger using MWCNT based nanofluids

The influence of different functional covalent groups on the thermophysical properties of carbon nanotube-base fluid was first investigated experimentally. To shed more light on this issue, cysteine (Cys) and silver (Ag) were covalently attached to the surface of the multi-walled carbon nanotubes (MWCNT). Functionality and morphology were studied by the aid of characterization instruments to confirm surface functionalization, as well. To calculate the thermal properties, different water-based nanofluids such as Gum Arabic-treated multi-walled carbon nanotubes (MWCNT-GA), functionalized MWCNT with cysteine (FMWCNT-Cys) and silver (FMWCNT-Ag) were employed as coolants to investigate the convection heat transfer coefficient, Nusselt number, friction loss, pressure drop and pumping power in a counter flow corrugated plate heat exchanger. Calculations were performed for Reynolds numbers ranging from 2500 to 10,000 (turbulent flow) and nanoparticle weight percentages of 0.0% to 1.0% using a FORTRAN code. Nanofluid properties were also measured through experimentation. It was found that increasing Reynolds number, Peclet number or fraction of nanomaterial would improve the heat transfer characteristics of the nanofluid. However, for a specific material, augmentation of Reynolds number or nanomaterial fraction would cause the required pumping power to rise but this penalty was relatively small. In all the investigated cases, heat transfer rate and power consumption were found to be less for water compared to nanofluids. Besides, heat removal in nanofluids was discovered to be higher than that of water for a specific pumping power. Therefore, performance of the plate heat exchanger can be enhanced by choosing MWCNT/water as the working fluid.     

Heat transfer and pressure drop correlations for compact heat exchangers with multi-region louver fins

An experimental study on the air-side heat transfer and pressure drop characteristics for brazed aluminum heat exchangers with multi-region louver fins and flat tubes was conducted. A series of tests were conducted for heat exchangers with different numbers of louver regions at the air-side Reynolds numbers of 400–1600 based on the louver pitch. The air-side thermal performance data were analyzed by using the effectiveness-NTU method. The characteristics of the heat transfer and pressure drop for heat exchangers with different geometry parameters were presented in terms of the Colburn j factor and Fanning friction f factor as function of the Reynolds number.     

Heat transfer and pressure drop correlations for the wavy fin and flat tube heat exchangers

A total of 11 cross-flow heat exchangers having wavy fin and flat tube were studied experimentally. A series of tests were conducted for air side Reynolds number in the range of 800–6500 with different fin pitches, fin lengths and fin heights, at a constant tube-side water flow rate of 2.5 m³/h. The air side thermal performance data were analyzed using the effectiveness-NTU method. The characteristics of heat transfer and pressure drop for different geometry parameters were reported in terms of Colburn j-factor and Fanning friction factor f, as a function of Re. The effects of fin pitch, fin height and fin length on the performance of heat transfer and pressure drop were examined. The general correlations for j and f factors were derived by multiple linear regression analysis and F test of significance. The correlations for j and f factors can predict 95% of the experimental data within ±10%.     

Experimental study of the effect of winglet location on heat transfer enhancement and pressure drop in fin-tube heat exchangers

Local heat transfer coefficients were measured on fin-tube heat exchanger with winglets using a single heater of 2 inch diameter and five different positions of winglet type vortex generators. The measurements were made at Reynolds number about 2250. Flow losses were determined by measuring the static pressure drop in the system. Results showed a substantial increase in the heat transfer with winglet type vortex generators. It has been observed that average Nusselt number increases by about 46% while the local heat transfer coefficient improves by several times as compared to plain fin-tube heat exchanger. The maximum improvement is observed in the re-circulation zone. The best location of the winglets was with ΔX = 0.5D and ΔY = 0.5D. The increase in pressure drop for the existing situation was of the order of 18%.     

Boiling heat transfer and pressure drop of ammonia-lithium nitrate solution in a plate generator

This paper presents the thermal and pressure-drop experimental evaluation of a fusion plate heat exchanger (PHE) during boiling conditions of a solution of lithium nitrate in ammonia. The data are representative of the working conditions of generators in single-effect absorption chillers. The solution flow rate and outlet temperature were modified in the ranges of 0.041–0.083 kg/s and 78–95 °C, respectively. Correlations for single-phase-flow heat transfer are used to characterize the boiling heat transfer. The influences of the heat flux, mass flux and exit-vapour quality are analyzed. Boiling heat-transfer coefficients and correlations for the Nusselt number are obtained. Results are compared with Cooper’s and Ayub’s correlations for boiling heat transfer. Pressure drop in the solution side was also measured and one correlation was obtained to characterize the frictional pressure drop under boiling conditions.     

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。     

Investigation of local heat transfer coefficients in plate heat exchangers with temperature oscillation IR thermography and CFD

A method for the measurement of local convective heat transfer coefficients from the outside of a heat-transferring wall has been developed. This method is contact-free and fluid independent, employing radiant heating by laser or halogen spotlights and an IR camera for surface temperature measurements; it allows for the rapid evaluation of the heat transfer coefficient distribution of sizable heat exchanger areas. The technique relies first on experimental data of the phase lag of the outer surface temperature response to periodic heating, and second on a simplified numerical model of the heat exchanger wall to compute the local heat transfer coefficients from the processed data. The IR temperature data processing includes an algorithm for temperature drift compensation, phase synchronization between the periodic heat flux and the measured temperatures, and Single Frequency Discrete Fourier Transformations. The ill-posed inverse heat conduction problem of deriving a surface map of heat transfer coefficients from the phase-lag data is solved with a complex number finite-difference method applied to the heat exchanger wall. The relation between the local and the mean heat transfer coefficients is illuminated, calculation procedures based on the thermal boundary conditions are given. The results from measurements on a plate heat exchanger are presented, along with measurements conducted on pipe flow for validation. The results show high-resolution surface maps of the heat transfer coefficients for a chevron-type plate for three turbulent Reynolds numbers, including a promising approach of visualizing the flow field of the entire plate. The area-integrated values agree well with literature data. CFD calculations with an SST and an EASM–RSM were carried out on a section of a PHE channel. A comparison with the measured data indicates the shortcomings of even advanced turbulence models for the prediction of heat transfer coefficients but confirms the advantages of EASM–RSM in complex flows.     

Investigation of heat transfer and pressure drop in a concentric heat exchanger with snail entrance

In this study, in order to increase heat transfer in concentric double-pipe heat exchangers by passive method, snail which is mounted at inlet of the inner pipe and assumed as a swirl generator was used. In the experimental set-up, cold air in ambient conditions was passed through the inner pipe while hot water was flowing through the annulus. The effects of a snail on the heat transfer and pressure drop were investigated for parallel and counter-flow, and obtained Nusselt numbers (Nu) were compared with those found, using a standard correlation such as Dittus–Boelter equation given for axial flows in smooth pipes. The results were correlated in the form of Nusselt number as a function of Reynolds number, Prandtl number and the swirling angle. An augmentation of up to 120% in Nusselt number was obtained in the swirl flow for counter-flow and 45° swirling angle. Though the swirl flow effect of the snail caused some increase in pressure drop, this effect was unimportant compared with the improvement in heat transfer capacity.     

Generalized correlations for predicting heat transfer and pressure drop in plate heat exchanger channels of arbitrary geometry

Characteristics of the flow in chevron plate heat exchangers are investigated through visualization tests of channels with β = 28° and β = 61°. Mathematical model is then developed with the aim of deriving correlations for prediction of f and Nu for flow in channels of arbitrary geometry (β and b/l). Thermal and hydraulic characteristics are evaluated using analytical solutions for the entrance and fully developed regions of a sinusoidal duct adapted to the basic single cell. The derived correlations are finally adjusted so as to agree with experimental results from tests on channels with β = 28° and β = 65°. f and Nu calculated by the presented correlations are shown to be consistent with experimental data from the literature at Re = 2–10,000, β = (15–67)° and b/l = 0.26–0.4.     

Condensation heat transfer and pressure drop of refrigerant R-410A flow in a vertical plate heat exchanger

Heat transfer and associated frictional pressure drop in the condensing flow of the ozone friendly refrigerant R-410A in a vertical plate heat exchanger (PHE) are investigated experimentally in the present study. In the experiment two vertical counter flow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60°. Downflow of the condensing refrigerant R-410A in one channel releases heat to the upflow of cold water in the other channel. The effects of the refrigerant mass flux, imposed heat flux, system pressure (saturated temperature) and mean vapor quality of R-410A on the measured data are explored in detail. The results indicate that the R-410A condensation heat transfer coefficient and associated frictional pressure drop in the PHE increase almost linearly with the mean vapor quality, but the system pressure only exhibits rather slight effects. Furthermore, increases in the refrigerant mass flux and imposed heat flux result in better condensation heat transfer accompanying with a larger frictional pressure drop. Besides, the imposed heat flux exhibits stronger effects on the heat transfer coefficient and pressure drop than the refrigerant mass flux especially at low refrigerant vapor quality. The friction factor is found to be strongly influenced by the refrigerant mass flux and vapor quality, but is almost independent of the imposed heat flux and saturated pressure. Finally, an empirical correlation for the R-410A condensation heat transfer coefficient in the PHE is proposed. In addition, results for the friction factor are correlated against the Boiling number and equivalent Reynolds number of the two-phase condensing flow.     

Heat transfer enhancement and pressure drop for fin-and-tube compact heat exchangers with wavy rectangular winglet-type vortex generators

In the current work, heat transfer enhancement and pressure loss penalty for fin-and-tube compact heat exchangers with the wavy-up and wavy-down rectangular winglets as special forms of winglet are numerically investigated in a relatively low Reynolds number flow. The rectangular winglets were used with a particular wavy form for the purpose of enhancement of air side heat transfer performance of fin-and-tube compact heat exchangers. The effect of Reynolds numbers from 400 to 800 and angle of attack of 30° of wavy rectangular winglets are also examined. The effects of using the wavy rectangular winglet, conventional rectangular winglet configuration and without winglet as baseline configuration, on the heat transfer characteristics and flow structure are studied and analyzed in detail for the inline tube arrangements. The results showed that the wavy rectangular winglet can significantly improve the heat transfer performance of the fin-and-tube compact heat exchangers with a moderate pressure loss penalty. In addition, the numerical results have shown that the wavy winglet cases have significant effect on the heat transfer performance and also, this augmentation is more important for the case of the wavy-up rectangular winglet configuration.     

Study of carbon dioxide condensation in chevron plate exchangers; pressure drop analysis

Condensation pressure drop of carbon dioxide in brazed plate heat exchangers was investigated, and is presented in this paper. Carbon dioxide is known as an environmental friendly refrigerant with an Ozone Depletion Potential (ODP) equal to zero and Global Warming Potential (GWP) equal to unity, and has favorable thermodynamic and transport properties though it requires higher operating pressures (～15–30 bar). Brazed-type plate heat exchangers that can withstand high pressure are a good choice for such applications. This paper presents the procedure, data collection, and results for three brazed plate heat exchangers with different inner geometries. The test exchangers showed good performance at high system pressures with reasonable pressure drops (less than 8%). The collected experimental data that covered real world operating conditions are valuable for the design of cascade condensers with carbon dioxide as the low-side refrigerant.     

Local heat transfer measurements of plate finned-tube heat exchangers by infrared thermography

An experimental study is performed using an infrared thermovision to monitor temperature distribution over a plate-fin surface inside the plate finned-tube heat exchangers. The differentiation of the temperature function is derived to determine the local convective heat transfer coefficients on the tested fin, using a local element lumped conduction equation included the convective effect on the boundaries with experimental data. It is disclosed that the infrared thermography is capable of rapidly detecting location and extent of transition and separation regions of the boundary layer over the whole surface of the tested models. Through the comparison of the test results on the strategy region of the in-line and staggered arrangements, it is more easy to understand or interpret the detailed dynamic phenomena of flow existed in the heat exchangers. In addition, the experimental results demonstrate that the averaged heat transfer coefficient of staggered configuration is 14-32% higher than that of in-lined configuration