Heat Transfer Enhancement Ratio Research Articles

Physical quantity synergy analysis and efficiency evaluation criterion of heat transfer enhancement

On the basis of the core-flow heat transfer enhancement and field synergy principle, the difference between fluid-oriented and surface-oriented heat transfer enhancement methods is performed. The physical nature of heat transfer enhancement and friction reduction is illustrated through revealing the synergy principle of heat transfer and friction characteristics of physical quantity and describing the internal relations of velocity and temperature fields as well as velocity and pressure fields. The efficiency evaluation criterion (EEC) and the efficiency evaluation plot are proposed to analyze and evaluate heat transfer enhancement techniques corresponding to the different regions in the efficiency evaluation plot. Region I represents the heat transfer enhancement ratio is larger than the pumping power increase ratio and region II represents the heat transfer enhancement ratio is less than the pumping power increase ratio in comparison with a bare tube. 3-D numerical simulations of several inserts in tube are provided to verify multi-fields synergy principle of convective heat transfer and efficiency evaluation plot is carried out to demonstrate the characteristics of enhancement tubes.

Experimental investigation of the effect of inclination angle on convection-driven melting of phase change material in a rectangular enclosure

This paper investigates the dynamic thermal behavior of phase change material (PCM) melting in a rectangular enclosure at various inclination angles. Lauric acid as a PCM with high Prandtl number (Pr≈100) is used. The enclosure is heated isothermally from one side while the other walls are thermally insulated. Experiments were performed with hot wall temperatures of 55, 60 and 70°C (3.6×108⩽Ra⩽8.3×108) for different inclination angles of 0°, 45° and 90°. Image processing of melt photographs along with recorded temperatures were used to calculate the melt fractions, Nusselt numbers and the local interfacial heat transfer rates at the solid–liquid interface. Qualitative time-dependent natural convection flow structures were deduced indirectly from the instantaneous shape of the solid–liquid interface which were confirmed by quantitative data from temperature measurements. The results reveal that the enclosure inclination has a significant effect on the formation of natural convection currents and consequently on the heat transfer rate and melting time of the PCM. As the inclination angle is decreased from 90° to 0°, the convection currents in the enclosure increases and chaotic flow structures appear. When melting commences in the horizontally inclined enclosure, the solid–liquid interface line becomes wavy which implies the formation of Benard convection cells in the liquid PCM. For the same hot wall temperatures, a decrease in inclination angle leads to a considerable enhancement in energy transport from the hot wall of the enclosure to the PCM. It is found that the heat transfer enhancement ratio for the horizontal enclosure is more than two times higher than that of the vertical enclosure.

English

In this study, the effects of equilateral triangular bodies placed in the channel on heat transfer and pressure drop characteristics were examined numerically. Reynolds number varied from 5000 to 10,000. The effects of the blockage ratio at constant edge length (B=20 mm) and Reynolds number of triangular bodies were investigated. An RNG based k-e method was used and a constant surface temperature was applied to the bottom wall. The numerical results were presented with respect to temperature contours, local and mean Nusselt number, local and mean surface friction factor changes and heat transfer enhancement ratios. The results showed that the presence of the equilateral obstacles in the flow field enhanced the heat transfer. Temperature distributions behind the bodies were affected by the position of the triangles and the Reynolds number and changed greatly in the vertical direction along the x distance beyond the bluff bodies. The intensity of the oscillations decreased with the increase of the Reynolds number. It was observed that for all cases overall heat transfer enhancement was provided. It was found that the highest values of OHTE were reached at W/B=0 and Re=5.0 and lowest were at W/B=1 and Re=10.0.

Convection Condensation Heat Transfer of Steam-Air Mixture With Heat Pipe Heat Exchanger

A heat pipe heat exchanger (HPHE) was used to investigate the heat transfer performance of steam-air mixture condensation, analogous to the dew condensation of flue gas, when the steam volume fraction ranged from 0 to 20%, and the inlet temperature of steam-air mixture varied from 70 to 120°C. Self-assembled monolayers (SAMs) treatment with n-octadecyl mercaptan was adopted to modify the condensation surface of the heat pipe. The comparisons were conducted to examine the influence of SAMs on condensation heat transfer of steam-air mixture vapor. The results indicate that the convection condensation heat transfer coefficient increases with the increase of steam volume fraction and Re number of the steam-air mixture. As the inlet temperature increases, the heat transfer coefficient decreases accordingly. The heat transfer performance can be improved by the SAMs surface, with a heat transfer enhancement ratio up to 1.6 at a condition of 20% of the steam volume fraction and 1500 Re number.

Heat transfer in a high Hartmann number MHD duct flow with a circular cylinder placed near the heated side-wall

The heat transfer from the side-wall of a duct through which an electrically conducting fluid flows within a strong transverse magnetic field is investigated. A circular cylinder aligned with the magnetic field is offset from the duct centerline. In this configuration the flow is well described by a quasi-two-dimensional model and is therefore solved on a two-dimensional domain. The effects of blockage ratio, gap ratio and Reynolds number on the flow and heat transfer are considered. An optimal cylinder position is determined using an efficiency index defined as the ratio of heat transfer enhancement to pressure drop penalty resulting from insertion of the cylinder in the channel. Cylinder placement with gaps to the heated wall of between 0.83 and 1.4 diameters performed best, achieving at least 95% of the peak efficiency indices for each blockage ratio. These gap ratios corresponded to the periodic shedding of alternating-sign vortices into the wake which interacted with the heated side-wall boundary layer to form counter-rotating vortex pairs carrying hot fluid into the duct interior. By offsetting the cylinder from the wake centreline, heat transfer enhancement of up to 48% compared to the centreline placement is achieved with only a modest increase in the pressure head losses from cylinder drag.

Investigation on heat transfer enhancement and pressure loss of double swirl chambers cooling

By merging two standard swirl chambers, an alternative cooling configuration named double swirl chambers (DSC) has been developed. In the DSC cooling configuration, the main physical phenomena of the swirl flow in swirl chamber and the advantages of swirl flow in heat transfer augmentation are maintained. Additionally, three new physical phenomena can be found in DSC cooling configuration, which result in a further improvement of the heat transfer: (1) impingement effect has been observed, (2) internal heat exchange has been enhanced between fluids in two swirls, and (3) “∞” shape swirl has been generated because of cross effect between two chambers, which improves the mixing of the fluids. Because of all these improvements, the DSC cooling configuration leads to a higher globally-averaged thermal performance parameter (Nu¯¯/Nu∞/(f/f0)1/3) than standard swirl chamber. In particular, at the inlet region, the augmentation of the heat transfer is nearly 7.5 times larger than the fully developed non-swirl turbulent flow and the circumferentially averaged Nusselt number coefficient is 41% larger than the standard swirl chamber. Within the present work, a further investigation on the DSC cooling configuration has been focused on the influence of geometry parameters e.g. merging ratio of chambers and aspect ratio of inlet duct on the cooling performance. The results show a very large influence of these geometry parameters in heat transfer enhancement and pressure drop ratio. Compared with the basic configuration of DSC cooling, the improved configuration with 20% to 23% merging ratio shows the highest globally-averaged thermal performance parameter. With the same cross section area in tangential inlet ducts, the DSC cooling channel with larger aspect ratio shows larger heat transfer enhancement and at the same time reduced pressure drop ratio, which results in a better globally-averaged thermal performance parameter.

Effect of space distance for boiling heat transfer on micro porous coated surface in confined space

This work provides an experimental analysis of the boiling heat transfer of methanol on plain and micro porous coated surfaces inside confined space. Three space confinements with distance of 1.0, 2.0, 3.0mm and an unconfined space were tested on plain and micro porous coating surfaces. Effects of space confinement, surface treatment and heat flux on the heat transfer coefficient and critical heat flux were discussed. From the test results, we may deduct that the boiling heat transfer performance in confined spaces was affected by four major effects, i.e. (a) vapor blowing and liquid suction effect, (b) thin film evaporation, (c) vapor leaving resistance and (d) partial dryout effect on plain surface. But only (a) and (c) are important on micro porous coating surface. The combination of these effects resulted in the micro porous to plain surfaces heat transfer enhancement ratio to have different characteristics at low, moderate and high heat flux conditions. Micro porous coating is a very effective boiling heat transfer enhancement treatment at low and moderate heat fluxes conditions. The enhancement ratio reduced in very narrow space confinement or at very high heat flux condition.

Experimental study of forced convective heat transfer in a different arranged corrugated channel

In this study, heat transfer rate for sinusoidal corrugated channel has been experimentally investigated. Three different type sharp corrugation peak fins and a plain surface were used in the experiment. Results were carried out for constant heat flux of 616W/m2, varied Reynolds number Re 1500 to 8000 for the corrugation angle (27, 50 and 22/60°) and channel height of 5 and 10mm. Nusselt number (Nu), convection heat transfer coefficient (h), Colburn factor (j) and enhancement ratio (E) against Reynolds number (Re) have been studied. The effects of the wavy geometry and channel height have been discussed. The increase of corrugated angle gave rise to a heat transfer rate.

Evaluation of Heat Transfer on Micro‐Structured Surfaces Based on Entropy Production

AbstractSurface structures with a well‐defined shape and dimension in a staggered arrangement were evaluated in terms of their capability to improve the single‐phase convective heat transfer in channels with small dimensions. Two evaluation criteria were used: (i) the thermal efficiency index, defined by the ratio of heat transfer enhancement and pressure drop increase, and (ii) the irreversible entropy generation which is determined by the entropy production rates through heat conduction and dissipation. Beside experimental measurements, parametric studies were performed using computational fluid dynamics to establish the entropy generation rates within the heated flow channel.

Condensation heat transfer on pin-fin tubes: Effect of thermal conductivity and pin height

New experimental data are reported for condensation of ethylene glycol and R-113 at near atmospheric pressure and low velocity on three identical pairs of pin-fin tubes made of copper, brass and bronze. The only geometric parameter varied was pin height. For both fluids tested, copper pin-fin tubes showed the highest gain in heat-transfer enhancement ratio with increasing pin height. Vapour-side, heat-transfer enhancement ratio was relatively weakly dependent on tube thermal conductivity for R-113 and more strongly dependant in the case of ethylene glycol. Data showed a strong heat-transfer enhancement dependence on unflooded area enhancement.

Analytical analysis of heat transfer and pumping power of laminar nanofluid developing flow in microchannels

Thermal management issues are limiting barriers to high density electronics packaging and miniaturization. Liquid cooling using micro and mini channels is an attractive alternative to large and bulky aluminum or copper heat sinks. These channels can be integrated directly into a chip or a heat spreader, and cooling can be further enhanced using nanofluids (liquid solutions with dispersed nanometer-sized particles) due to their enhanced heat transfer effects reported in literature. The goals of this study are to evaluate heat transfer improvement of a nanofluid heat sink with developing laminar flow forced convection, taking into account the pumping power penalty. The phrase heat transfer enhancement ratio (HTR) is used to denote the ratio of average heat transfer coefficient of nanofluid to water at the same pumping power. The proposed model uses semi-empirical correlations to calculate nanofluid thermophysical properties. The predictions of the model are found to be in good agreement with experimental studies. The validated model is used to identify important design variables (Reynolds number, volume fraction and particle size) related to thermal and flow characteristics of the microchannel heat sink with nanofluids. Statistical analysis of the model showed that the volume fraction is the most significant factor impacting the HTR, followed by the particle diameter. The impact of the Reynolds number and other interaction terms is relatively weak. The HTR is maximized at smallest possible particle diameter (since smaller particles improve heat transfer but do not impact pumping power). Then, for a given Reynolds number, an optimal value of volume fraction can be obtained to maximize HTR. The overall aim is to present results that would be useful for understanding and optimal design of microchannel heat sinks with nanofluid flow.

Thermal performance of undulant narrow channel with skewed sinusoidal wall-waves and 45° ribs

This experimental study examines the properties of heat transfer, pressure drop and thermal performance factor (TPF) of a narrow undulant channel enhanced by skew sinusoidal wall-waves and 45° ribs. Detailed Nusselt number (Nu) distributions over the wavy ribbed channel wall are generated by the steady-state infrared thermography method and examined along with the accompanying pressure drop coefficient (f) in the test Reynolds number (Re) range of 800–9000. With references to the Nusselt numbers (Nu∞) and pressure drop coefficients (f∞) of smooth plain tube, the area-averaged heat transfer enhancement (HTE) ratios, pressure drop augmentations (f/f∞) and TPF for the tested undulant channel are compared with those obtained from other passive HTE devices. Empirical correlations evaluating the area-averaged Nusselt number (Nu¯) over the undulant ribbed channel wall and the pressure drop coefficients are generated to assist design activities using this newly devised HTE measure.

Heat transfer correlation models for electrospray evaporative cooling chambers of different geometry types

Abstract Development of future electronics for high speed computing requires a silent thermal management method capable of dissipating a broad range of heat generated from application-specific integrated circuits, while keeping the skin temperature below 45 °C. Electrospray evaporative cooling (ESEC) chambers show promise because of their ability to dissipate a broad range of heat within a relatively small size. However, the development and the optimization of ESEC chambers are currently restricted, in part due to the lack of sufficient empirical heat transfer correlations. This paper investigates empirical heat transfer correlations for ESEC chambers with three different geometry types. Since the unstable multi-jet behavior of an ESEC chamber is similar to that of a free-surface traditional impinging liquid jet, these correlations are based on the traditional impinging liquid jet’s empirical correlations, yet are modified to factor in the electric field effect. The results show that the heat transfer enhancement ratio correlations and the Nusselt number correlations for different ESEC chambers cover more than 83% of the experimental data, within ±10% deviation. The sensitivity analysis results and experimental data prove that the variation in the enhancement ratio is sensitive to that of the potential and the flow rate. It is not sensitive to the geometric factor of the same ESEC type. This paper presents a natural convection correlation for chip-scale, heated, flat surfaces when the Rayleigh number is below 3000. Further investigation is necessary to extend these heat transfer correlations to cover additional parameters for different thermal management applications.

Experimental and numerical investigation of impingement heat transfer on a flat and micro-rib roughened plate with different crossflow schemes

A combined experimental and numerical investigation of the heat transfer characteristics within an array of impingement jets on a flat and micro-rib roughened plate has been conducted. The experiments are carried out in a perspex model using a transient liquid crystal method. Local jet temperatures are measured at several positions on the impingement plate to account for an exact evaluation of the heat transfer coefficient. The effects of variation of jet-to-plate spacing, crossflow schemes and jet Reynolds number on the distribution of the local Nusselt number and the related pressure loss are investigated experimentally. The jet-to-plate spacing H/ d = 3 results in the highest heat transfer coefficients for both the flat and the micro-rib roughened plate. Overall heat transfer performance on the micro-rib roughened plate is always best for the minimum crossflow case. The heat transfer enhancement ratio increases with increasing Reynolds numbers. All the data from the micro-rib roughened plate is compared to that of the flat plate. The measurements are complemented by a numerical part, in which one particular case from the experiments is investigated by means of a commercial Computational Fluid Dynamics (CFD) software package. In the numerical results, for the heat transfer pattern the same trend as in the experiments is found. In the comparison of local heat transfer coefficients, results from experiments and predictions are of the same order. Significantly more local details are captured in the CFD analysis. The numerical study also addresses and compares the effects of 1D and 3D heat transfer by accounting for the solid domain of the rib material (conjugate heat transfer). The results of the latter show that the CFD predictions can be regarded as a valuable complement to experimental measurements for configurations where heat transfer on rib roughened surfaces are to be evaluated.

열전달 촉진 표면에서 임계 열유속까지의 물의 풀 비등 열전달계수

In this work, nucleate pool boiling heat transfer coefficients(HTCs) of pure water are measured on horizontal 26 fpi low fin, Turbo-B and Thermoexcel-E square surfaces of 9.53 mm length. HTCs are taken from 10 <TEX>$kW/m^2$</TEX> to critical heat flux for all surfaces. Test results show that critical heat fluxes(CHFs) of all enhanced surfaces are greatly improved as compared to that of a plain surface. CHFs of water on the 26 fpi low fin surface, Thermoexcel-E surface, and Turbo-B are increased up to 320%, 275%, and 150% as compared to that of the plain surface, respectively. CHF of the Turbo-B enhanced surface is lower than that of the 26 fpi low fin surface due to the surface geometry. The heat transfer enhancement ratios of the Thermoexcel-E surface, low fin surface and Turbo-B enhanced surface are 1.6~2.9, 1.6~2.1, 1.4~1.7 respectively in the range of heat fluxes tested. Judging from these results, it can be said that these types of enhanced surfaces can be used in heat transfer applications at high heat fluxes.

Heat transfer characteristics of falling film evaporation on horizontal tube arrays

A falling film heat transfer test facility has been built for the measurement of falling film evaporation in a vacuum of about 1000 Pa. At this condition, only convective evaporation occurred in the liquid film. The Reynolds numbers of falling film over a range from 21.6 to 108.1 were tested on six-tube arrays made of enhanced or smooth tubes. Results show that the tubes with both enhanced outer and inner surfaces give high heat flux. Besides, as the Reynolds number increases, the heat transfer enhancement ratio of falling film evaporation decreases. A semi-analytical correlation is established to predict the heat transfer coefficients of falling film evaporation on smooth tube arrays, considering the contributions of partially dryout and fully wet regimes, respectively. For enhanced tubes, the heat transfer enhancement ratios to the smooth tubes were also correlated.

Numerical Simulation of Turbulent Flow and Heat Transfer in an Anti- Gravity Blind Duct with Tangential Entry Jet

This numerical study investigates the flow and heat transfer characteristics in a vertical square blind duct with the coolant fed by a tangential entry jet. The detailed Nusselt number (Nu) distributions over the five constituent walls of the blind duct are calculated at duct (jet) Reynolds numbers Re(Rej) of 5000(20000), 7000(28000) and 10000(40000) us- ing the commercial CFD Star CD code. As an attempt to explore the buoyancy effect on heat transfer performances, three different heat fluxes, which vary the gravitational Grashof numbers (Grg) at the fixed Re, are imposed on each duct wall to vary the buoyancy levels. The jet-induced flow phenomena in the blind duct exhibit various heat transfer impacts on the five duct walls over which the different near-wall flow structures are generated. This is demonstrated by cross-examining the detailed Nu distributions and the area-averaged Nusselt number ( Nu ) over the five duct walls at the tested Re and Grg. The cross-plane swirls induced by the tangential entry jet together with the impinging jet flows considerably elevate the Heat Transfer Enhancement (HTE) performances in the blind duct. Within the parametric conditions simulated, the ratios of Nu to the Dittus-Boelter levels (Nu� ) over the jet wall, back wall, impingement wall, side wall and end wall are re- spectively raised to 3-5.7, 2.8-5.6, 3-5.8, 2.7-4.8 and 3.1-6.1; while the HTE ratios ( Nu /Nu� ) over these duct walls con- sistently decrease as Re increases.

Periodically Fully-Developed Flow and Heat Transfer Over Flat and Oval Tubes Using a Control Volume Finite-Element Method

A Control Volume Finite-Element Method in conjunction with imposed periodically fully-developed flow conditions was used to perform a two-dimensional, laminar, steady-flow numerical study comparing the performance of a flat tube and an oval tube to that of a round tube in a simulated heat exchanger device for the case of specified heat flux along the tube walls. The Reynolds number range for the study was 50 to 350. Fluids of Prandtl number 0.7 and 7.0 were considered. For the cases studied, the heat transfer enhancement ratio was less than one indicating that the round tube outperformed both the flat tube and the oval tube based on heat transfer considerations alone. However, for all cases studied, the heat transfer performance ratio was greater than one indicating that if both heat transfer performance and required pumping power are considered, both the flat tube and oval tube outperformed the round tube.

Heat transfer augmentation by swirl generators inserted into a tube with constant heat flux

A novel conical injector type swirl generator ( CITSG) is devised in this study. Performances of heat transfer and pressure drop in a pipe with the CITSG are experimentally examined for the CITSGs' angle ( α) of 30°, 45° and 60° in Reynolds number ( Re) range of 10,000–35,000. Moreover, circular holes with different numbers ( N) and cross-section areas ( A h) are drilled on the CITSG. In this way, total areas ( A t = N · A h) of the holes on the CITSG are equaled each other. Besides, flow directors having three different angles ( β = 30°, 60° and 90°) to radial direction are attached to every one of the holes. This study is a typical example for decaying flow. All experiments were conduced with air accordingly; Prandtl number was approximately fixed at 0.71. The local Nusselt number ( Nu x ), heat transfer enhancement ratio ( Nu ER) and heat transfer performance ratio ( Nu PR) are calculated and discussed in this paper. It is found that the Nu ER decreases with increase in Reynolds number, the director angle ( β), the director diameter ( d), and with decrease in the CITSG angle ( α). Likewise, variation of Nu PR and Nu ER is also essentially similar for the same independent parameters.

Evaporation heat transfer enhancement of R-134a flowing inside smooth and micro-fin tubes using the electrohydrodynamic technique

Heat transfer enhancement using the electrohydrodynamic (EHD) technique during evaporation inside horizontal smooth and micro-fin tubes is experimentally investigated. The test section is a counter-flow concentric tube-in-tube heat exchanger with R-134a flowing inside inner tube and heating water flowing in the annulus. Smooth and micro-fin inner tubes are used as test section. The outer diameter and length of inner tubes are 9.52 mm and 2.50 m, respectively. The outer tube is a smooth copper tube having inner diameter of 27 mm. The electrode is made from stainless steel rod of 1.47 mm diameter. Experiments are performed at average saturation temperatures ranging between 10 and 20 °C, heat fluxes ranging between 10 and 20 kW/m 2 at mass flux of 400 kg/m 2 s and applied voltage of 2.5 kV. The maximum heat transfer enhancement ratios are 1.25 and 1.15 for smooth tube and micro-fin tube, respectively. New correlations based on the data gathered during this work for predicting the evaporation heat transfer coefficient in smooth tube and micro-fin tube are proposed for practical applications.