Improvement of heat transfer with perforated circular holes in finned tubes of air-cooled heat exchanger

The present study investigated the effect of perforated circular finned-tube (PCFT) on the convective heat transfer performance of circular finned-tube heat exchangers. The air-side convective heat transfer coefficients increased by 3.55% and 3.31% for 2-hole and 4-hole PCFT cases, respectively. The increase in the convective heat transfer coefficient was related to the reduction of the recirculation region by introducing the perforations at the flow-separation locations on the finned tube. The pressure drop across the finned-tube bundles increased by 0.68% and 2.08% for the 2-hole PCFT and 4-hole PCFT cases, respectively. The greater pressure drop in the case of the 4-hole PCFT might be due to excessive flow disturbances produced by multiple perforations. The fin factor defined as the ratio of the % increase of the convective heat transfer coefficient and that of the pressure drop was 5.19 for the 2-hole PCFT case, whereas that was 1.59 for the 4-hole PCFT case.     

Enhancement of shell side forced convective heat transfer in the shell tube‐type heat exchanger using thin plate‐type supports

The shell side heat transfer and pressure drop in counterflowing water were experimentally investigated on the basis of the overall heat transfer coefficient. The investigation was intended to identify ways to get higher performance for the cooler in a BWR nuclear power plant. The following three conclusions were reached in the study. (1) Predicted performance of the heat exchanger, using the overall heat transfer coefficient based on the outside area of the tube K_o, indicated an enhancement by 92% compared with the measured performance of the conventional segmental baffle‐type heat exchanger. (2) The tube side pressure drop ΔP_t=20 kPa and the shell side pressure drop ΔP_s=70 kPa were obtained, and were within the allowable value ΔP_a=80 kPa. The shell side pressure drop of the low‐pressure drop spacer could be decreased by 20% as compared with that of the standard spacer. (3) The enhancement constant of the shell side heat transfer using the low‐pressure drop spacer was about 1.2 times as large as that of the standard spacer, regardless of the pumping power. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 455–471, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10097     

Influence of transverse vibrations on convective heat transfer in parallel flow tube-in-tube heat exchanger

Tube-in-tube heat exchangers are widely used in food processing industries and wastewater treatment for both heating and cooling. Enhancement techniques namely active, passive, and compound are developed to reduce the thermal resistance in heat exchangers by improving convective heat transfer with or without increase in surface area. The present experimental study is aimed at analyzing the influence of vibrations on the convective heat transfer of a parallel flow tube-in-tube heat exchanger. The heat exchanger is placed in horizontal position and is subjected to transverse vibrations under turbulent fluid flow conditions. Experiments were performed at four frequencies (20, 40, 60, and 100 Hz), three amplitudes (1, 2, and 3 m/s²), and three vibration generator positions along its length, in the Reynolds number range of 10 710 to 21 420. An enhancement in Nusselt number is found with vibration than without vibration throughout the entire range of Reynolds numbers. A maximum enhancement of 33% at 40 Hz frequency, 3 m/s² amplitude, and vibration generator position at three-fourth of the tube length was observed. Empirical correlations are developed for Nusselt number to determine the heat transfer coefficient with vibration with an error of $\pm$10%.     

Cooling enhancement in an air-cooled finned heat exchanger by thin water film evaporation

A theoretical analysis on the cooling enhancement by applying evaporative cooling to an air-cooled finned heat exchanger is presented in this work. A two-dimensional model on the heat and mass transfer in a finned channel is developed adopting a porous medium approach. Based on this model, the characteristics of the heat and mass transfer are investigated in a plate-fin heat exchanger with the interstitial surface fully covered by thin water film. Assuming that the Lewis number is unity and the water vapor saturation curve is linear, exact solutions to the energy and vapor concentration equations are obtained. The cooling effect with application of evaporative cooling was found to be improved considerably compared with that in the sensible cooler. This is because the thermal conductance between the fin and the air increases due to the latent heat transfer caused by the water evaporation from the fin surface. It is also found that the cooling enhancement depends greatly on the fin thickness. If the fin is not sufficiently thick, the cooling enhancement by the evaporative cooling decreases since the fin efficiency drops considerably due to the water evaporation from the fin surface. The fin thickness in the evaporative cooler should be increased larger than that in the sensible cooler to take full advantage of the cooling enhancement by the water evaporation.     

Turbulent heat transfer enhancement in a heat exchanger using helically corrugated tube

The augmentation of convective heat transfer in a single-phase turbulent flow by using helically corrugated tubes has been experimentally investigated. Effects of pitch-to-diameter ratio (P/D_H = 0.18, 0.22 and 0.27) and rib-height to diameter ratio (e/D_H = 0.02, 0.04 and 0.06) of helically corrugated tubes on the heat transfer enhancement, isothermal friction and thermal performance factor in a concentric tube heat exchanger are examined. The experiments were conducted over a wide range of turbulent fluid flow of Reynolds number from 5500 to 60,000 by employing water as the test fluid. Experimental results show that the heat transfer and thermal performance of the corrugated tube are considerably increased compared to those of the smooth tube. The mean increase in heat transfer rate is between 123% and 232% at the test range, depending on the rib height/pitch ratios and Reynolds number while the maximum thermal performance is found to be about 2.3 for using the corrugated tube with P/D_H = 0.27 and e/D_H = 0.06 at low Reynolds number. Also, the pressure loss result reveals that the average friction factor of the corrugated tube is in a range between 1.46 and 1.93 times over the smooth tube. In addition, correlations of the Nusselt number, friction factor and thermal performance factor in terms of pitch ratio (P/D_H), rib-height ratio (e/D_H), Reynolds number (Re), and Prandtl number (Pr) for the corrugated tube are determined, based on the curve fitting of the experimental data.     

Investigations of convective heat transfer in ferrofluid microflows using lattice-Boltzmann approach

By using the lattice-Boltzmann method, the mesoscopic models for the ferrofluid are developed to simulate flow and thermal processes of the ferrofluid flowing through a micro channel. The models include a variety of forces and potentials acting on the ferrofluid system as well as heat exchange between the suspended magnetic nanoparticles and the ambient liquid. Some numerical examples are given to discuss the enhancement or suppression mechanism of heat transfer of the ferrofluid by adjusting the orientation and magnitude of the magnetic field gradient.     

Effect of cross-flow on the performance of air-cooled heat exchanger fans

In large-scale applications such as arrays of axial fans in air-cooled heat exchanger systems, edge–proximity and wind-induced cross-flow may decrease the flow through some fans by causing the flow to enter them at off-axis angles. In this study, such off-axis inflows were introduced by inserting inlet pipe sections between the plenum chamber of a standard test facility and one of three different scale model test fans of 1542 mm diameter. Fan power consumption turned out to be completely independent of off-axis inflow angle up to 45°. Fan total-to-total pressure rise was found to be independent of off-axis inflow angle, and the decrement in fan pressure rise was equal to the dynamic pressure based on the cross-flow velocity component at the fan inlet. Analysis showed that for model fans to represent the cross-flow behaviour of their prototypes, they should have the same ratio of dynamic pressure to pressure rise, and the same dimensionless characteristic slope at their operating points. The performance of a row of fans operating at off-axis inflow conditions representing a cooling system was well predicted by a simple model assuming that the fans farther from the edges induce cross-flows over the fans closer to the edges.     

Prediction of nanofluid convective heat transfer using the dispersion model

The laminar convective heat transfer behavior of nanofluids through a straight tube is numerically investigated in this paper. A new mechanism which is proposed to explain considerable enhancement of nanofluids heat transfer is dispersion that intends to consider the irregular movements of the nanoparticles. Applying this additional mechanism leads to promising results in comparison with the predictions by traditional homogenous model with effective properties. To validate the dispersion model results, the experimental results for three kinds of nanofluids are used. Also the effect of nanoparticle size on nanofluid heat transfer is examined. The obtained results show good agreement between the theoretical and experimental results.     

Experimental investigation of single phase convective heat transfer coefficient in a corrugated plate heat exchanger for multiple plate configurations

Corrugated plate heat exchangers have larger heat transfer surface area and increased turbulence level due to the corrugations. In this study, experimental heat transfer data are obtained for single phase flow (water-to-water) configurations in a commercial plate heat exchanger for symmetric 30°/30°, 60°/60°, and mixed 30°/60° chevron angle plates. Experiments were carried out for Reynolds number ranging from 500 to 2500 and Prandtl number from 3.5 to 6.5. Experimental results show significant effect of chevron angle and Reynolds number on the heat transfer coefficient. Based on the experimental data, a correlation to estimate Nusselt number as a function of Reynolds number, Prandtl number and chevron angle has been proposed.     

Free convective heat transfer in an enclosure with an internal louvered blind

A numerical study has been conducted of free convection in a tall vertical enclosure with an internal louvered metal blind. The study considers the effects of Rayleigh number, enclosure aspect ratio, and blind geometry on the convective heat transfer. The numerical model has been validated against experimental measurements and the results have been presented in terms of an empirical correlation for the average Nusselt number. The correlation is applicable to an enclosure with an internal metal blind. It has been shown that the Nusselt number correlation can be combined with a simple one-dimensional model to closely predict the enclosure U-value.     

Enhancement of natural convective heat transfer from tall,vertical heated plate to water

An enhancement technique was developed for natural convection heat transfer from a tall, vertical heated plate to water. Rectangular grid fins attached to the base plate were utilized as a heat transfer promoter. These grid fins redirect the high‐temperature fluid ascending along the base plate toward the outside of the boundary layer and introduce the low‐temperature ambient fluid toward the base plate instead. The heat transfer coefficients of thus‐treated surfaces were measured and compared with a nontreated surface and a surface with conventional vertical plate‐fins. The highest performance was achieved for the experimental surfaces. In particular, the experimental surfaces with 5‐mm‐high, nonconducting grids and with 10‐mm‐high, conducting grid fins show 27% and 80% higher heat transfer coefficients compared to the turbulent heat transfer coefficients of the nontreated surface, respectively. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(2): 178–190, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10083     

Enhancement of nucleate boiling heat transfer using carbon nanotubes

The effect of carbon nanotubes (CNTs) on nucleate boiling heat transfer in R22 and water is investigated with the addition of 1.0 vol% of CNTs. Test results showed that CNTs increase boiling heat transfer coefficients of these fluids. Especially, large enhancement up to 28.7% was observed at low heat fluxes of less than 30 kW/m². With increasing heat flux, however, the enhancement was suppressed due to vigorous bubble generation. Fouling on the surface was not observed during this study. Optimum quantity and type of CNTs and their dispersion should be examined for their commercialization in power plants and refrigeration equipment.     

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.     

Laminar convective heat transfer in chaotic configuration

The convective heat transfer in chaotic configuration of circular cross-section under laminar flow regime at different values of Dean number and Prandtl number is investigated numerically. The chaotic configuration is the combination of 90° bends and coils. The insertion of equidistant 90° bends between the two consecutive coil produces the phenomenon of flow inversion. The hydrodynamics and heat transfer under laminar flow conditions in the chaotic configuration with constant wall flux as a boundary condition is studied. The control-volume finite difference method with second-order accuracy is used. The chaotic configuration shows a 25–36% enhancement in the heat transfer due to chaotic mixing while relative pressure drop is 5–6%. The effect of Prandtl number on fully developed heat transfer coefficient is also reported. It is observed that heat transfer increases with increase in Prandtl number. The stretching and folding phenomenon in chaotic configuration is observed and discussed for heat transfer coefficient and pressure drop in the chaotic configuration. The cyclic oscillation behavior in the heat transfer coefficient with downstream distance in the chaotic configuration and coiled tube is also observed and discussed. It appeared that heat transfer is strongly influenced by flow inversion. The effect of boundary conditions on heat transfer performance in the chaotic configuration as well as in the coiled tube is also carried out. The study is further extended to predict hydrodynamics and heat transfer with temperature-dependent viscosity in the chaotic configuration. A comparative study for heat transfer and friction factor is also carried out for constant and temperature-dependent viscosity in coiled tube and chaotic configuration. It was observed that the heat transfer under heating condition with temperature-dependent viscosity is higher as compared to the constant viscosity result while friction factor shows the reverse phenomenon in the chaotic configuration.     

Transient heat transfer in a U-tube borehole heat exchanger

A transient heat transfer model has been development for a thermal response test (TRT) on a vertical borehole with a U-tube. Vertical borehole heat exchangers are frequently coupled to ground source heat pumps, which heat and cool buildings. The model provides an analytical solution for the vertical temperature profiles of the circulating fluid through the U-tube, and the temperature distribution in the ground. The model is verified with data sets from a laboratory sandbox and field TRTs, as well as a previously reported numerical solution. Unlike previous analytical models, the vertical profiles for the circulating fluid are generated by the model without any assumption of their functional form.     

Optimization principles for convective heat transfer

Optimization for convective heat transfer plays a significant role in energy saving and high-efficiency utilizing. We compared two optimization principles for convective heat transfer, the minimum entropy generation principle and the entransy dissipation extremum principle, and analyzed their physical implications and applicability. We derived the optimization equation for each optimization principle. The theoretical analysis indicates that both principles can be used to optimize convective heat-transfer process, subject to different objectives of optimization. The minimum entropy generation principle, originally derived from the heat engine cycle process, optimizes the convective heat-transfer process with minimum usable energy dissipation focusing on the heat–work conversion. The entransy dissipation extremum principle however, originally for pure heat conduction process, optimizes the heat-transfer process with minimum heat-transfer ability dissipation, and therefore is more suitable for optimization of the processes not involving heat–work conversion. To validate the theoretical results, we simulated the convective heat-transfer process in a two-dimensional foursquare cavity with a uniform heat source and different temperature boundaries. Under the same constraints, the results indicate that the minimum entropy production principle leads to the highest heat–work conversion while the entransy dissipation extremum principle yields the maximum convective heat-transfer efficiency.     

An experimental study on the quantitative interpretation of local convective heat transfer for a plate fin and tube heat exchanger using the lumped capacitance method

An experimental study has been performed to investigate the heat transfer characteristics of a plate fin and tube heat exchanger. Existing transient and steady methods are inappropriate for the measurement of heat transfer coefficients of the thin heat transfer model. In this study, the lumped capacitance method based on liquid crystal thermography was adopted. The method is validated through impinging jet and plate flow experiments. The two experiments showed very good agreements with those of the well-known transient method with the thick acryl model. And the lumped capacitance method showed similar results regardless of the thickness of the polycarbonate model if the Bi of the fin is small enough. The method was also applied for the heat transfer coefficient measurements of a fin and tube heat exchanger. Quantitative heat transfer coefficients of the plate fin were successfully obtained.     

Enhancement of natural convection heat transfer from an inclined heated plate using rectangular grids

The effect of an inclination angle on the natural convection heat transfer from an inclined heated plate with rectangular grids is investigated. Heat transfer coefficients are measured in air when the plates are inclined at angles from ?30 to +60 from a vertical plane, grid heights are in the range of 5 to 10 mm, and diagonal lengths of the grid are 25, 50, 100, and 200 mm. For each configuration, the surface heat flux ranges from 50 to 200 W/m². It is found that the rectangular grids increase local heat transfer coefficients when the grids are applied to an inclined plate. The rectangular grids increase the average heat transfer coefficients along the horizontal centerline of the plate by up to 20% compared to those coefficients of a smooth plate, even when the angles of inclination are ±30° © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(5): 408–419, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10043     

Forced convective heat transfer of nanofluids in microchannels

Convective heat transfer coefficient and friction factor of nanofluids in rectangular microchannels were measured. An integrated microsystem consisting of a single microchannel on one side, and two localized heaters and five polysilicon temperature sensors along the channel on the other side were fabricated. Aluminum dioxide (Al₂O₃) with diameter of 170 nm nanofluids with various particle volume fractions were used in experiments to investigate the effect of the volume fraction of the nanoparticles to the convective heat transfer and fluid flow in microchannels. The convective heat transfer coefficient of the Al₂O₃ nanofluid in laminar flow regime was measured to be increased up to 32% compared to the distilled water at a volume fraction of 1.8 volume percent without major friction loss. The Nusselt number measured increases with increasing the Reynolds number in laminar flow regime. The measured Nusselt number which turned out to be less than 0.5 was successfully correlated with Reynolds number and Prandtl number based on the thermal conductivity of nanofluids.     

Single-phaseexperimental analysis of heat transfer in helically finned heat exchanger

A methodology for designing helically serrated finned tube heat exchanger based on the logarithmic Mean Temperature Difference (LMTD) method is validated with experimental tests. The method uses semi-empirical correlations for calculating convective coefficients both inside and outside staggered tube bundles. Equipment was designed, built, and installed in a paper factory in order to validate the methodology. Comparisons between predictions and experimental data show a precision of approximately 96% in heat transfer and approximately 90% in pressure drop for Reynolds numbers upper to 10,000.