Prediction of heat transfer coefficient on the fin inside one-tube plate finned-tube heat exchangers

The finite difference method in conjunction with the least-squares scheme and the experimental temperature data is proposed to predict the average heat transfer coefficient and the fin efficiency on the fin inside one-tube plate finned-tube heat exchangers for various air speeds and the temperature difference between the ambient temperature and the tube temperature. Previous works showed that the heat transfer coefficient on this rectangular fin is very non-uniform. Thus the whole plate fin is divided into several sub-fin regions in order to predict the average heat transfer coefficient and the fin efficiency on the fin from the knowledge of the fin temperature recordings at several selected measurement locations. The results show that the surface heat flux and the heat transfer coefficient on the upstream region of the fin can be markedly higher than those on the downstream region. The fin temperature distributions depart from the ideal isothermal situation and the fin temperature decreases more rapidly away from the circular center, when the frontal air speed increases. The average heat transfer coefficient on the fin increases with the air speed and the temperature difference between the ambient temperature and the tube temperature. This implies that the effect of the temperature difference between the tube temperature and the ambient temperature is not negligent.     

Air side heat transfer coefficients of discrete plate finned-tube heat exchangers with large fin pitch

The objective of this study is to investigate the heat transfer characteristics of discrete plate finned-tube heat exchangers with large fin pitches. Thirty-four heat exchangers were tested with variations of fin pitches, the number of tube rows, fin alignment, and vertical fin space. The j-factor of the discrete plate finned-tube exchanger was analyzed as a function of coil geometry and then compared with that of the continuous plate finned-tube heat exchanger. For fin pitches of 7.5–15 mm, the j-factors of the discrete plate finned-tube heat exchangers were 6.0–11.6% higher than those of the continuous plate finned-tube heat exchangers. Two separate correlations for the j-factor were developed for the inline and the staggered fin alignment in the discrete plate finned-tube heat exchangers to predict the measured data within a relative deviation of 2.9%.     

An experimental study in determining the local heat transfer coefficients for the plate finned-tube heat exchangers

A three-dimensional inverse problem in determining the local heat transfer coefficients for the plate finned-tube heat exchangers utilizing the steepest descent method (SDM) and a general purpose commercial code CFX4.4 is applied successfully in the present study based on the measured temperature distributions on fin surface by infrared thermography.Two different tube arrangements (i.e. in-line and staggered) with different fin pitch and air velocity are considered and the corresponding local heat transfer coefficients are to be determined. Results show that some interesting phenomena of the local heat transfer coefficients for the finned surface are found in the work and the averaged heat transfer coefficient of the staggered configuration is about 8–13% higher than that of the in-line configuration.     

A three-dimensional inverse problem in imaging the local heat transfer coefficients for plate finned-tube heat exchangers

A three-dimensional inverse heat conduction problem in imaging the local heat transfer coefficients for plate finned-tube heat exchangers utilizing the steepest descent method and a general purpose commercial code CFX4.4 is applied successfully in the present study based on the simulated measured temperature distributions on fin surface by infrared thermography.It is assumed that no prior information is available on the functional form of the unknown local heat transfer coefficients in the present study. Thus, it can be classified as function estimation for the inverse calculations.Two different heat transfer coefficients for in-line tube arrangements with different measurement errors are to be estimated. Results show that the present algorithm can obtain the reliable estimated heat transfer coefficients.     

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.     

Thermofluid characteristics of frosted finned-tube heat exchangers

The performance of frosted finned-tube heat exchangers of different fin types is investigated by experiments in this paper. The effects of the air flow rate, the air relative humidity, the refrigerant temperature, and the fin type on the thermofluid characteristics of the heat exchangers are discussed. The time variations of the heat transfer rate, the overall heat transfer coefficient, and the pressure drop of the heat exchangers are presented. The heat transfer rate, the overall heat transfer coefficient, and the pressure drop for heat exchangers with re-direction louver fins are higher than those with flat plate fins and one-sided louver fins are. The amount of frost formation is the highest for heat exchangers with re-direction louver fins.     

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.     

Local effects of longitudinal heat conduction in plate heat exchangers

In a plate heat exchanger, heat transfer from the hot to the cold fluid is a multi-dimensional conjugate problem, in which longitudinal heat conduction (LHC) along the dividing walls often plays some role and can not be neglected. Large-scale, or end-to-end, LHC is always detrimental to the exchanger’s effectiveness. On the contrary, if significant non-uniformities exist in the distribution of either convective heat transfer coefficient, small-scale, or local, LHC may actually enhance the exchanger’s performance by improving the thermal coupling between high heat transfer spots located on the opposite sides of the dividing wall.     

Accuracy of one-dimensional design procedures for finned-tube heat exchangers

In design procedures for finned-tube heat exchangers a common simplification is assuming that the temperature distribution is one-dimensional. In this way, the heat exchanger can be schematized as a thermal circuit with three thermal resistances in series: internal convection to the tube, conduction through the tube wall, and external convection through the fin assembly. The aim of this work is to quantitatively evaluate the accuracy of one-dimensional schematizations in the context of finned-tube heat exchangers utilized in air-conditioning applications. To this purpose, first three-dimensional benchmark results are obtained employing an in-house FEM code. Afterwards, a simplified two-dimensional model is proposed and validated through a comparison with the three-dimensional results. Finally, the simplified two-dimensional model and the commercial software COMSOL Multiphysics are used to conduct a parametric study aimed at assessing the accuracy of one-dimensional schematizations. The main conclusion is that the accuracy of one-dimensional design procedures is quite acceptable for practical purposes, since it leads to errors in the estimation of heat flow rates that are always less than 2%.     

Frequency response and spatial resolution of a thin foil for heat transfer measurements using infrared thermography

A technique for measuring the spatio-temporal distribution of convective heat transfer has been developed using a test surface fabricated from a thin foil heated electrically. If the heat capacity of the test surface is sufficiently low, the fluctuating temperature on the foil can be measured using high-frame-rate infrared thermography. This method, however, has an inherent problem in that the temperature on the test surface attenuates both in time and space due to thermal inertia and conduction. In the present study, the frequency response and the spatial resolution of a thin foil were examined analytically considering heat losses. In order to derive general relationships, non-dimensional variables of fluctuating frequency and spatial wavenumber were introduced to formulate the temporal and spatial amplitudes of the temperature on the test surface. Based on these relationships, the upper limits on the detectable fluctuating frequency and spatial wavenumber were successfully formulated using governing parameters of the measurement system. This enables us to evaluate quantitatively the reliability of the heat transfer measurement by infrared thermography. The values, evaluated here for the practical conditions, indicated that this measurement technique is promising for investigating the spatio-temporal behavior of heat transfer caused by flow turbulence.     

Heat transfer enhancement using hybrid nanofluids in spiral plate heat exchangers

A possible way to enhance the rate of heat transfer in the spiral plate heat exchanger (SPHE) is by employing hybrid nanofluids as its working medium. Hence, in the present work, effects of hybrid nanofluids on the thermal performance of SPHE has been investigated numerically. First, a countercurrent SPHE is designed and modeled. Later, simulation of SPHE has been carried out by employing conventional fluid , nanofluids , and hybrid nanofluids to investigate the heat transfer rates. Finally, the performance of SPHE using hybrid nanofluid is compared with that of using water and nanofluids. The heat transfer augmentation of approximately 16%‐27% with hybrid nanofluids of overall 4% nanoparticles volume concentration and 10%‐16% with 2% nanoparticles volume concentration is observed when compared with that of pure water. Therefore, it can be inferred that the application of hybrid nanofluids in SPHE seems to be one of the promising solutions for augmentation of its thermal performance.     

Dimensional analysis for the heat transfer characteristics in the corrugated channels of plate heat exchangers

Using the Buckingham Pi theorem, this study derives dimensionless correlations to characterize the heat transfer performance of the corrugated channel in a plate heat exchanger. The experimental data are substituted into these correlations to identify the flow characteristics and channel geometry parameters with the most significant influence on the heat transfer performance. Simplified correlations by omitting the factors with less influence are then obtained. The results show that Nu_x is affected primarily by Re, R/D_h, x/D_h, and β. Neglecting the minor effect of factors on Nu_x, it is shown that Nu_m is determined primarily by Re, R/D_h and β.     

Investigation of heat transfer and pressure drop in plate heat exchangers having different surface profiles

It would be misleading to consider only cost aspect of the design of a heat exchanger. High maintenance costs increase total cost during the services life of heat exchanger. Therefore exergy analysis and energy saving are very important parameters in the heat exchanger design. In this study, the effects of surface geometries of three different type heat exchangers called as PHE_flat (Flat plate heat exchanger), PHE_corrugated (Corrugated plate heat exchanger) and PHE_asteriks (Asterisk plate heat exchanger) on heat transfer, friction factor and exergy loss were investigated experimentally. The experiments were carried out for a heat exchanger with single pass under condition of parallel and counter flow. In this study, experiments were conducted for laminar flow conditions. Reynolds number and Prandtl number were in the range of 50 ? Re ? 1000 and 3 ? Pr ? 7, respectively. Heat transfer, friction factor and exergy loss correlations were obtained according to the experimental results.     

Modeling of turbulent heat transfer and thermal dispersion for flows in flat plate heat exchangers

In this paper, heat transfer and dispersion for both laminar and turbulent regimes in heat exchangers and nuclear cores are considered. Such hydraulic systems might be seen as spatially periodic porous media. The existence of a turbulent flow within a porous medium structure suggests the use of a spatial average operator, combined to a statistical average operator. Previous works [M.H.J. Pedras, M.J.S. De Lemos, Macroscopic turbulence modeling for incompressible flow through undeformable porous media, Int. J. Heat Mass Transfer 44 (2001) 1081–1093; F. Kuwahara, A. Nakayama, H. Koyama, A numerical study of thermal dispersion in porous medium, J. Heat Transfer 118 (1996) 756–761] have applied a double average procedure to the thermal balance equation, which led to a macroscopic turbulent transport and a subsequent macro-scale equation featuring dynamic dispersion. Considering the heat flux at the solid surfaces as a boundary condition for the fluid energy balance, the model proposed in this paper allows one to take into account this dispersion as the sum of two contributions. The first one is the classical dispersion due to velocity heterogeneities [G. Taylor, Dispersion of solute matter in solvent flowing slowly through a tube, Proc. Roy. Soc. Lond. A 219 (1953) 186–203] and the second one is due to wall heat transfer. Applying Whitaker up-scaling method [S. Whitaker, Theory and applications of transport in porous media: the method of volume averaging, Kluwer Academic Publishers, 1999], a “closure problem” is then derived for a representative elementary volume, using the so-called Boussinesq approximation to account for small scale turbulence. The model is used to compute macro-scale heat transfer properties for turbulent flows inside a flat plate heat exchanger. It is shown that, for such flows, both dispersive fluxes strongly predominate over the macroscopic turbulent heat flux.     

A new method for heat transfer and fluid flow performance simulation of plate heat exchangers

Successful numerical simulation on heat transfer and fluid flow performances of plate heat exchangers is vital. Their complex structures often make the numerical calculation quite difficult and time-consuming. Conclusions drawn by the present work are promising for greatly simplifying the simulation. Different types of plates consisting of different numbers of periods are analyzed and it is concluded that the Nusselt number remains constant for different periods of different plates under different inlet velocities. The central friction coefficients behave the same as Nusselt number. For the first and last periods, the respective friction coefficient also remains for different plates. A small plate fraction with four periods is enough for performance prediction of any-sized plates.     

Study of carbon dioxide condensation in chevron plate exchangers; heat transfer analysis

The experimental investigation of carbon dioxide condensation in brazed plate heat exchangers is the main objective of this study. The current level of concern for the environment is at an all time high, therefore, it is important to look into methods and resources that lead to a cleaner and healthier future for the planet. This study details one such effort to reach this goal, focusing on condensation of carbon dioxide as a natural refrigerant in refrigeration systems. Three brazed plate heat exchangers with different geometry, each consisting of three channels, are tested. This paper focuses on the two-phase analysis, where carbon dioxide was the working fluid, flowing through the middle channel, and dynalene brine, the cooling fluid, flowed through the side channels of each geometry. Condensation of carbon dioxide occurred at saturation temperatures ranging from ?17.8 °C to ?34.4 °C and heat fluxes spanning 2.5–15.7 kW/m². An in-depth dimensional analysis was completed on the two-phase data yielding heat transfer correlations. Relationships of the two-phase heat transfer characteristics are presented, the data are compared with related studies, and conclusions are made from the two-phase data.     

Optimization of finned-tube heat exchangers for diesel exhaust waste heat recovery using CFD and CCD techniques

In this paper, response surface methodology (RSM) based on central composite design (CCD) is applied to obtain an optimization design of finned type heat exchangers (HEX) to recover waste heat from the exhaust of a diesel engine. The design is performed for a single point operation (1600 rpm and 60 N m) of an OM314 diesel engine obtained from experimental measurements. Based on the CCD principle, fifteen HEX cases with different fins height, thickness and number are modeled numerically and the optimization is done to have the maximum heat recovery amount and minimum of pressure drop along the heat exchanger.