Local Heat Transfer Enhancement Research Articles

Influence of Short Helical Tape on Local Convective Heat Transfer and Thermal Performance Characteristics in Turbulent Decaying Swirl Flow

This paper reports the experimental investigation of local convective heat transfer enhancement, flow friction and thermal performance factor behaviors in the tube fitted with the short helical tapes (SHTs) acting as decaying swirl flow generators. The tapes with three different helical tape angles (? = 90°, 135° and 180°) and three different channel numbers (N = 2, 3 and 4 channels) were tested under the uniform wall heat flux condition. The performance of each tape is compared with the performance of the plain tube subject to the same pumping power. The experimental results show that the heat transfer rates and friction factors of the tube with SHTs are respectively in range of 1.15 to 1.9 and 1.49 to 2.31 times of those in the plain, corresponding to thermal performances between 0.98 and 1.46. The correlations for Nusselt number (Nu) as a function of Reynolds number (Re), Prandtl number (Pr), helical tape angle (?) and the number of channel (N) are also developed.

Heat Transfer Characteristics and Reynolds Stress Budgets in Single-Rib Mounting Channel

Heat transfer and fluid flow in a single-rib mounting channel were investigated by directly solving Navier-Stokes and energy equations. Flow and thermal fields were considered to be fully developed at the inlet of the channel, and the simulation was made for spatial advancement of turbulent heat transfer. Keeping the frictional Reynolds number, Reτ0, at 150, the rib height ratio was changed in four steps from H/δ = 0.05 to H/δ = 0.4. Computational results were confirmed to be nearly independent of grid meshes. In addition, numerical accuracy was confirmed through close agreement between computed mean pressure and the experiment by Yao et al. (1995). The numerical results revealed that the highest value of the mean Nusslet number was as large as 1.3 times the smooth surface consuming the same pumping power, and the local enhancement of heat transfer was correlated with the turbulence increase near the rib front and the reattachment point. According to the Reynolds stress budgets for H/δ = 0.2, there were mechanisms to induce powerful fluctuations: (1) Streamwise fluctuation was increased through production by flow deceleration in the upstream of the rib; (2) Redistribution to wall-normal and spanwise fluctuations was fortified by the fluid splattering to the rib front. Therefore, excellent performance of heat transfer was concluded to occur due to flow structures, which induce the strong disturbance near the rib front triggering smooth transition of the separated shear layer.

The Influence of the Magnitude of Gravitational Acceleration on Marangoni Convection About an Isolated Bubble under a Heated Wall

Thermocapillary or Marangoni convection is the liquid motion caused by surface tension variation in the presence of a temperature gradient along a gas–liquid or vapor–liquid interface. This work numerically investigates the effect of the magnitude of gravitational acceleration on the flow and temperature fields resulting from the presence of a hemispherical air bubble of constant radius of 1.0 mm, situated on a heated wall immersed in a liquid silicone oil layer of constant depth of 5.0 mm. The model is oriented such that the Marangoni and gravitational forces act to oppose one another. To elucidate the effect of gravity on Marangoni flow and heat transfer, the simulations were carried out for a silicone oil of Prandtl number 83, at a Marangoni number of 915. The gravity levels tested were 0g, 0.01g, 0.1g, 0.25g, 0.5g, 0.75g, and 1g, where g represents the earth gravitational acceleration of 9.81 m/s 2 . The influence of the magnitude of gravitational acceleration on the velocity profile along the bubble interface and on the location of maximum velocity was analyzed. It was found that the gravity level affects the velocity profile by influencing the interfacial temperature gradient, but that the location of maximum velocity was almost independent of gravity level. The increase in heat flux on the wall to which the bubble is attached was calculated and it has been determined that local heat transfer enhancement of up to nearly 1.7 times that of the conduction only case can be achieved for the parameter range tested. Furthermore, local enhancement was observed to occur up to a distance of seven bubble radii for the zero-gravity case, but increased gravity levels cause a reduction in the effective radius of enhancement. The influence of the Marangoni flow on the heat transfer for the opposite cooled wall has also been analyzed.

Experimental and Numerical Investigation of Impingement Cooling in a Combustor Liner Heat Shield

A combined experimental and numerical investigation of the heat transfer characteristics inside an impingement cooled combustor liner heat shield has been conducted. Due to the complexity and irregularity of heat shield configurations, standard correlations for regular impingement fields are insufficient and detailed investigations of local heat transfer enhancement are required. The experiments were carried out in a perspex model of the heat shield using a transient liquid crystal method. Scaling of the model allowed to achieve jet Reynolds numbers of up to Rej=34,000 without compressibility effects. The local air temperature was measured at several positions within the model to account for an exact evaluation of the heat transfer coefficient. Analysis focused on the local heat transfer distribution along the heat shield target plate, side rims, and central bolt recess. The results were compared with values predicted by a standard correlation for a regular impingement array. The comparison exhibited large differences. While local values were up to three times larger than the reference value, the average heat transfer coefficient was approximately 25% lower. This emphasized that standard correlations are not suitable for the design of complex impingement cooling pattern. For thermal optimization the detailed knowledge of the local variation of the heat transfer coefficient is essential. From the present configuration, some concepts for possible optimization were derived. Complementary numerical simulations were carried out using the commercial computational fluid dynamics (CFD) code ANSYS CFX. The motivation was to evaluate whether CFD can be used as an engineering design tool in the optimization of the heat shield configuration. For this, a validation of the numerical results was required, which for the present configuration was achieved by determining the degree of accuracy to which the measured heat transfer rates could be computed. The predictions showed good agreement with the experimental results, both for the local Nusselt number distributions as well as for averaged values. Some overprediction occurred in the stagnation regions, however, the impact on overall heat transfer coefficients was low and average deviations between numerics and experiments were in the order of only 5–20%. The numerical investigation showed that contemporary CFD codes can be used as suitable means in the thermal design process.

Numerical assessment of local forcing on the heat transfer in a turbulent channel flow

The influence of local forcing on an incompressible turbulent channel flow is numerically investigated. The extensive information provided by the direct numerical simulations enables us to have a better understanding of the physical mechanism responsible for local heat transfer enhancement. Time-periodic blowing/suction is applied by means of thin spanwise slots located at the lower and upper walls. The molecular Prandtl number is 0.71 and the Reynolds number based on the wall friction velocity and the channel half-height, Reτ, is 394 for the unforced case. The normal perturbing velocity is varied sinusoidally in time at several perturbing frequencies between 0.16<f¯<1.6 or 0.011<f+<0.11 and at a fixed amplitude of Ao=0.2. A phase-averaging procedure is employed to discriminate between the coherent and incoherent fluctuating components. It is shown that coherent thermal fluctuations reach peak values near the forcing slot, then sharply decay and almost disappear in a short distance downstream. The incoherent thermal fluctuations also show peak values next the source; however, they decay downstream to resemble the incoherent fluctuations of the unperturbed channel. It was concluded that the forcing frequency of f¯=0.64 or f+=0.044 produced the largest local increase in the skin friction in the region 0.1<x/Lx<0.3 (where Lx is the channel length), followed by the highest augmentation of the Stanton number. It is found that augmentation of the wall shear stress fluctuations is the major cause of skin friction, wall heat flux, and Stanton number enhancement downstream from the local forcing source. On the other hand, local maxima of Reynolds shear stresses, wall-normal turbulent heat fluxes, and the incoherent component of streamwise vorticity fluctuations exhibited analogous behavior along the streamwise direction.

Circumferential temperature distribution during nucleate pool boiling outside smooth and modified horizontal tubes

In the work an approach to avoid a circumferential temperature distribution existing during nucleate pool boiling on a horizontal cylinder within low heat flux densities is presented. The idea of the approach is local heat transfer enhancement by a porous layer application on a part of the heating surface. An experiment on nucleate pool boiling heat transfer from horizontal cylinders to saturated R141b and water under atmospheric pressure is reported. Experiments have been conducted using stainless steel tubes with the outside diameter between 8 mm and 23 mm with the active length of 250 mm. The outside surface of the tubes was smooth or partially coated with a porous metallic layer. In particular, measurements of inside circumferential temperature distribution have been performed.

Local heat transfer enhancement around a matrix of wall-mounted cubes using passive flow control: Large-eddy simulations

The purpose of the research reported in this paper was to investigate the influence of attaching vortex generators (VGs) to a surface of a heated cube on flow structures and heat transfer using large-eddy simulation (LES). The cube was located in the middle of a matrix of similar cubes. Two kinds of vortex generators were investigated. The first kind was a simple rib extending in the span-wise direction (VG1) while the second consisted of several of small cubes (VG2). The flow and heat transfer around a cube with vortex generators were compared with the flow and heat transfer around a smooth cube. The LES results showed that the flow in the boundary layer around the cubes with VGs is more turbulent and unsteady than the flow around the smooth cube. More complex structures are generated close to the surface of the cube with VGs, resulting in a considerable increase in the heat transfer coefficient. Local overheating was found behind the rib-shape VG while even distribution of temperature was observed over the surfaces of the cube in the VG2 case. There was a 14% and 17% global increase in the heat transfer coefficient in the VG1 and VG2 cases, respectively.

Application of temperature-sensitive paint for surface temperature measurement in heat transfer enhancement applications

This paper introduces the use of temperature-sensitive paint (TSP) to obtain the whole-field temperature distributions in heat transfer enhancement applications. Knowing the surface temperatures in such applications is important to quantify the heat transfer, as well as to obtain insight into the flow physics. Applying the TSP to a flat plate with and without a vortex generator showed that the TSP Nusselt number distribution over the flat plate alone matched the Blasius solution with less than 10% error. The TSP temperature distribution of the flat plate with the delta wing vortex generator showed a local heat transfer enhancement of about 30% and the locations of temperature extremes. This paper also provides a detailed guideline for the application, image acquisition, and image processing procedures for TSP along with its calibration surfaces.

A Numerical Study of Flow and Heat Transfer Enhancement Using an Array of Delta-Winglet Vortex Generators in a Fin-and-Tube Heat Exchanger

This work is aimed at assessing the potential of winglet-type vortex generator (VG) “arrays” for multirow inline-tube heat exchangers with an emphasis on providing fundamental understanding of the relation between local flow behavior and heat transfer enhancement mechanisms. Three different winglet configurations in common-flow-up arrangement are analyzed in the seven-row compact fin-and-tube heat exchanger: (a) single–VG pair; (b) a 3VG-inline array (alternating tube row); and (c) a 3VG-staggered array. The numerical study involves three-dimensional time-dependent modeling of unsteady laminar flow (330⩽Re⩽850) and conjugate heat transfer in the computational domain, which is set up to model the entire fin length in the air flow direction. It was found that the impingement of winglet redirected flow on the downstream tube is an important heat transfer augmentation mechanism for the common-flow-up arrangement of vortex generators in the inline-tube geometry. At Re=850 with a constant tube-wall temperature, the 3VG-inline-array configuration achieves enhancements up to 32% in total heat flux and 74% in j factor over the baseline case, with an associated pressure-drop increase of about 41%. The numerical results for the integral heat transfer quantities agree well with the available experimental measurements.

Heat Transfer, Pressure Loss and Flow Field Measurements Downstream of Staggered Two-Row Circular and Elliptical Pin Fin Arrays

This paper presents the results of heat transfer, total pressure loss, and wake flow field measurements downstream of two-row staggered elliptical and circular pin fin arrays. Two different types of elliptical fins are tested, i.e., a Standard Elliptical Fin (SEF) and a fin that is based on NACA four digit symmetrical airfoil shapes (N fin). The results are compared to those of a corresponding circular pin fin array. The minor axis lengths for both types of elliptical fins are kept equal to the diameter of the circular fins. Experiments are performed using Liquid Crystal Thermography and total pressure probe wake surveys in a Reynolds number range of 18 000 and 86 000 as well as Particle Image Velocimetry (PIV) measurements at ReD=18 000. The pin fins had a height-to-diameter ratio of 1.5. The streamwise and the transverse spacings were equal to one circular fin diameter, i.e., S/D=X/D=2. For the circular fin array, average Nusselt numbers on the endwall within the wake are about 27% higher than those of SEF and N fin arrays. Different local heat transfer enhancement patterns are observed for elliptical and circular fins. In terms of total pressure loss, there is a substantial reduction in case of SEF and N fins. The loss levels for the circular fin are 46.5% and 59.5% higher on average than those of the SEF and N fins, respectively. An examination of the Reynolds analogy performance parameter show that the performance indices of the SEF and the N fins are 1.49 and 2.0 times higher on average than that of circular fins, respectively. The thermal performance indices show a collapse of the data, and the differences are much less evident. Nevertheless, N fins still show slightly higher thermal performance values. The wake flow field measurements show that the circular fin array creates a relatively large low momentum wake zone compared to the SEF and N fin arrays. The wake trajectories of the first row of fins in circular, SEF and N fin arrays are also different from each other. The turbulent kinetic energy levels within the wake of the circular fin array are higher than those for the SEF and the N fin arrays. The transverse variations in turbulence levels correlate well with the corresponding local heat transfer enhancement variations.

정지된 상류의 원형실린더 사이의 편심률이 후방실린더의 열전달에 미치는 영향

The influence of eccentric(=staggeredness) ratio between stationary upstream circular cylinders on heat transfer characteristics of a heated downstream circular cylinder installed ill a channel was investigated experimentally In order to enhance the heat transfer rate of the heated downstream cylinder surface, we have changed the configuration of upstream cylinder As a result, we were able to obtain local time-averaged convective heat transfer enhancement of the heated cylinder by the relative replacement of upstream cylinder This is basically attributed to the mean flow structure change, such as flow separation, vortex shedding, and recirculation of the upstream cylinder including the reattachment and new thermal boundary developed at the downstream cylinder which are the results of the increase of the staggeredness ratio.

The enhancement of steam condensation heat transfer in a horizontal shell and tube condenser by addition of ammonia

This paper reports on the rates of condensation heat transfer for weak ammonia–water mixtures in a horizontal, shell and tube condenser. It is shown that for inlet ammonia concentrations in the range 0.2–0.9 wt.% the average condensation heat transfer for the condenser was enhanced by up to 14%. Furthermore, local enhancement of the condensation heat transfer of up to 34% occurred at local bulk vapour concentrations between 0.2 and 2 wt.% ammonia. This enhancement was caused by the Marangoni effect which produced a disturbed, turbulent banded condensate film and a corresponding drop in the thermal resistance of the condensate film.

Experimental Study of Surface-Mounted Obstacle Effects on Heat Transfer Enhancement by Using Transient Liquid Crystal Thermograph

Effects of surface-mounted obstacles on the local heat transfer enhancement of a base plate are investigated by using transient liquid crystal thermograph technique. To explore the geometry effects of short obstacles, the height less than one hydraulic diameter (d), three cross-sectional shapes of obstacles, i.e., circular, square and diamond, with variations in number of obstacles, obstacle spacing, and free-stream Reynolds number are considered. The maximum number of the obstacles in tandem array is 3 and the spacing between obstacles is 1d, 2d, or 4d. The free-stream Reynolds number ranges from 2100 to 4200. The experimental results reveal that the local heat transfer enhancement in front of leading circular and square obstacles are better than the diamond one, while the influenced area by the obstacle of the diamond shape is most remarkable. The present results disclose that an intermediate height (0.5d) of the protruding elements is more beneficial to the heat transfer enhancement in wake of the obstacle. With the sweepback leading edge of the top surface, the diamond and circular obstacles produce vortical flow across the obstacles and thus enhance heat transfer downstream in wake. Increasing Reynolds number leads to an enhancement in heat transfer performance. The number of and the spacing between the obstacles in tandem array are also influential factors to the flow structure and heat transfer enhancement on the basic plate.

Numerical simulation for heat and fluid characteristics of square duct with discrete rib turbulators

Attachment of rib turbulators to flow passages is one of the popular means of heat transfer enhancement. Typical use of the rib turbulators is found for example in the serpentine cooling air channel for the internal cooling of the gas turbine blades. Many studies therefore have been carried out for the heat transfer in ribbed channels and various types of ribs have been tested. A standard case of full-span ribs attached perpendicularly to the flow direction has been the first target and its friction and heat transfer characteristics have been reported in the references [1–3]. Further studies were made for other cases of attaching oblique ribs and V-shaped ribs to the channel wall. In these cases, secondary flow is incurred in the channel and enhances the fluid mixing between the near wall and core regions in the duct, eventually resulting in the enhancement of local heat transfer at the channel walls [4–6]. Discrete ribs also drew attention with which heat transfer enhancement due to the incurred secondary flow is achieved paying smaller pressure loss penalty [7– 10]. In relation with the existence of the spanwise gaps, secondary flows are generated downstream their spanwise edges. Generation of the secondary flows in addition to the flow separation and reattachment makes the phenomenon quite complicated with the discrete ribs. Detailed studies of the flow structure and characteristics of the related heat transfer are required but such studies have scarcely been made so far. In the present article, some results of the three-dimensional numerical computation conducted for flow and thermal fields over two types of array of ribs attached to a channel wall will be presented. One is of fullspan ribs and the other is of discrete ribs, where both arrays are attached in a position perpendicular to the flow direction. The discussion will be given to the effects of three-dimensionality and unsteadiness of the flow and thermal fields for twofold purposes, one to test the applicability of numerical study to the type of flow under concern and another to provide hints to experimental works to be made in future.

Fluid Flow and Heat Transfer in a Rotating Two-Pass Square Duct With In-Line 90-deg Ribs

Abstract Laser-doppler velocimetry and transient thermochromic liquid crystal measurements are presented to understand local fluid flow and surface heat transfer distributions in a rotating ribbed duct with a 180 deg sharp turn. The in-line 90-deg ribs were arranged on the leading and trailing walls with rib height-to-hydraulic diameter ratio and pitch-to-height ratio of 0.136 and 10, respectively. The Reynolds number, based on duct hydraulic diameter and bulk mean velocity, was fixed at 1.0×104 whereas the rotational number varied from 0 to 0.2. Results are compared with those of the rotating smooth duct flow in terms of maximum streamwise mean velocities Umax/Ub and turbulence intensities u′max/Ub, skewness of mean velocity profiles, secondary flow pattern, turn-induced separation bubble, and turbulence anisotropy. Nusselt number ratio mappings are also provided on the leading and trailing walls. The relationships between the fluid flow and local heat transfer enhancement are also documented. It is found that the rotating ribbed duct flow provides higher Umax/Ub,u′max/Ub, and stronger total averaged secondary flow and, hence heat transfer is enhanced. Comparisons with heat transfer data published by other research groups are also made. Furthermore, simple linear correlations between regional averaged Nusselt number ratio and rotation number are developed.

An experimental investigation of heat transfer in pin fin array

AbstractDetailed heat transfer measurements were performed by using 178 thermocouples in a channel with pin fin array. Local heat transfer coefficients and local heat transfer enhancement coefficients were obtained for eight Reynolds numbers ranging from 2000 to 100,000 on the endwall of the channel. The endwall boundary conditions for heat transfer investigation are heating the bottom endwall and heating symmetrically the bottom and top endwalls with constant heat flux. The mechanism of heat transfer enhancement with pin fin array has been discussed. © 2001 Scripta Technica, Heat Trans Asian Res, 30(7): 533–541, 2001

The effect of nozzle aspect ratio on stagnation region heat transfer characteristics of elliptic impinging jet

The local heat transfer characteristics of an elliptic impinging jet on a heated flat plate were experimentally investigated for various nozzle aspect ratios. Five elliptic nozzles of aspect ratio ( AR) of 1, 1.5, 2, 3 and 4 having the same equivalent diameter ( D e) were tested at jet Reynolds number of 10,000 to study the effect of nozzle aspect ratio on the local heat transfer enhancement in the stagnation region. The temperature distributions on the heated flat plate were measured using a thermochromic liquid crystal thermometry with a digital image processing system. The flow structure was visualized using a smoke-wire technique to get a better understanding on the impinging elliptic jet. With varying the nozzle-to-plate spacing, the isothermal contours on the heated flat plate showed the axis-crossover in heat transfer due to the self-induced curvature variation of a three-dimensional jet structure. For smaller nozzle-to-plate spacing ( L/D e <4 ), as the nozzle aspect ratio AR of elliptic jet increases, the heat transfer rate was increased larger than that for the axisymmetric jet ( AR = 1) in the stagnation region. This was mainly attributed to the engulfing large entrainment and enhanced large-scale mixing of the elliptic jet. The stagnation point Nusselt number was correlated for the nozzle aspect ratio and the nozzle-to-plate spacing as Nu o ∝ AR −0.082 L/D e −0.077 .

Mist/Steam Cooling in a Heated Horizontal Tube—Part 2: Results and Modeling

Experimental studies on mist/steam cooling in a heated horizontal tube have been performed. Wall temperature distributions have been measured under various main steam flow rates, droplet mass ratios, and wall heat fluxes. Generally, the heat transfer performance of steam can be significantly improved by adding mist into the main flow. An average enhancement of 100 percent with the highest local heat transfer enhancement of 200 percent is achieved with 5 percent mist. When the test section is mildly heated, an interesting wall temperature distribution is observed: The wall temperature increases first, then decreases, and finally increases again. A three-stage heat transfer model with transition boiling, unstable liquid fragment evaporation, and dry-wall mist cooling has been proposed and has shown some success in predicting the wall temperature of the mist/steam flow. The PDPA measurements have facilitated better understanding and interpreting of the droplet dynamics and heat transfer mechanisms. Furthermore, this study has shed light on how to generate appropriate droplet sizes to achieve effective droplet transportation, and has shown that it is promising to extend present results to a higher temperature and higher pressure environment. [S0889-504X(00)02502-2]

Heat transfer and flow field in a pipe with sinusoidal wavy surface—I. Numerical investigation

The majority of publications in the field of convective transport enhancement in conduits with wavy walls have provided the distribution of the mean Sherwood or Nusselt number per wavelength. The mechanisms, however, driving the increase in heat and mass transfer have not been clearly understood so far. This paper presents the results of a detailed numerical investigation of local heat and mass transfer enhancement in a pipe with sinusoidally varying diameter, covering a wide range of Reynolds numbers from laminar to turbulent flow. The discussion is focused on the predicted flow field and the turbulence structure, allowing a better understanding of the calculated Sherwood and Nusselt numbers. Part II of this paper deals with the experimental validation of the numerically achieved results.

Heat transfer enhancement by a particle on the bottom of a flume

Infrared thermography was used to study the effect of a spherical particle on the local heat transfer from a heated plate. The measurement of the temperature field with the particle and without it showed that the heat transfer coefficient increased locally by a factor of 1.5–3, for a particle of the order d p + = 10–25. A general decrease in the wall temperature was observed in the region under and in front of the particle. No effect of the particle material was noted. The temperature fluctuations at a point on the heated plate were also studied. These fluctuations are related to the bursting process in the turbulent flow. The bursting period increased by 60–70% for the flow near the particle, whereas the root mean square of the temperature fluctuations decreased up to 2.5 times. This may mean that the heat transfer enhancement is governed by a mechanism other than the change in the turbulence structure. Numerical computations of the flow field near the spherical particle were performed. These clearly indicate that fluid from the mainstream is directed towards the bottom of the flume, in front of the particle, thus cooling the heated surface. One can conclude that the change in the velocity profile near the wall caused by the particle is the reason for the local heat transfer enhancement.