Showing papers on "Heat transfer enhancement published in 1989"

Natural Convection Along a Vertical Wavy Surface With Uniform Heat Flux

Abstract: Laminar free convection along a semi-infinite vertical wavy surface has been studied by Yao (1983) for the case of uniform surface temperature. This is a model problem for the investigation of heat transfer from roughened surfaces in order to understand heat transfer enhancement. In many applications of practical importance, however, the surface temperature is nonuniform. In this note, the case of uniform surface heat flux rate, which is often approximated in practical applications and is easier to measure in a laboratory, has been investigated. Numerical results have been obtained for a sinusoidal wavy surface. The results show that the Nusselt number varies periodically along the wavy surface. The wavelength of the Nusselt number variation is half of that of the wavy surface, while the amplitude gradually decreases downstream where the boundary layer grows thick. It is hoped that experimental results will become available in the near future to verify the results of this investigation.

An Experimental Investigation of Heat Transfer Coefficients in a Spanwise Rotating Channel With Two Opposite Rib-Roughened Walls

Abstract: Liquid crystals are used in this experimental investigation to measure the heat transfer coefficient in a spanwise rotating channel with two opposite rib-roughened walls. The ribs (also called turbulence promoters or turbulators) are configured in a staggered arrangement with an angle of attack to the mainstream flow, α, of 90° for all cases. Results are presented for three values of turbulator blockage ratio, e/Dh (0.1333, 0.25, 0.333) and for a range of Reynolds numbers from 15,000 to 50,000 while the test section is rotated at different speeds to give Rotational Reynolds numbers between 450 and 1800. The Rossby number range is 10 to 100 (Rotation number of 0.1 to 0.01). The effect of turbulator blockage ratios on heat transfer enhancement is also investigated. Comparisons are made between the results of geometrically identical stationary and rotating passages of otherwise similar operating conditions. The results indicate that a significant enhancement in heat transfer is achieved in both the stationary and rotating cases, when the surfaces are roughened with turbulators. For the rotating case, a maximum increase over that of the stationary case of about 45% in the heat transfer coefficient is seen for a blockage ratio of 0.133 on the trailing surface in the direction of rotation and the minimum is a decrease of about 6% for a blockage ratio of 0.333 on the leading surface, for the range of rotation numbers tested. The technique of using liquid crystals to determine heat transfer coefficients in this investigation proved to be an effective and accurate method especially for nonstationary test sections.Copyright © 1989 by ASME

The Influence of Turbine Clearance Gap Leakage on Passage Velocity and Heat Transfer Near Blade Tips: Part II—Source Flow Effects on Blade Suction Sides

Abstract: A study has been conducted to investigate influences of tip leakage flow on heat transfer and flow development along the pressure side of a gas turbine blade. An analysis of the sink character of the flow situation indicates that high velocities and accelerations are generated very near the gap, and an apparatus was specifically designed to model the phenomena and to permit resolution of the expected localized near-gap heat transfer enhancement

Augmentation of laminar forced-convective heat transfer by the application of a transverse electric field

Abstract: The effect of a uniform d-c electric field on laminar forced-convective heat transfer has been studied experimentally with a weakly conducting fluorocarbon refrigerant in the liquid state that is flowing in a channel confined by parallel-plate electrodes, one of which serves as a heat transfer surface of uniform heat flux. The dependencies of the heat transfer coefficient and the pressure drop on the sign and the magnitude of an applied voltage, the heat flux at the heat transfer surface, the electrical conductivity of the test fluid, etc. are presented, and the structure and the mechanism of the electroconvection causing the heat transfer enhancement are considered.

Heat Transfer Visualization and Measurement in Unstable Concave-Wall Laminar Boundary Layers

Abstract: Using liquid crystal sheet in a hybrid constant-wall-temperature/constant-heat-flux procedure, heat transfer distributions have been measured in concave-wall laminar boundary layers with a natural Gortler vortex system in a near-zero pressure gradient. Stanton numbers at vortex downwash positions across the span exceeded those at upwash positions by factors as high as three. Spanwise-averaged Stanton numbers exceeded analytical flat-plate values only after the appearance of highly inflected upwash velocity profiles and the onset of span wise meandering of the vortices, where the Gortler number exceeded ten. Levels then reached values comparable with turbulent correlations, at Reynolds numbers and turbulence levels (up to 3 percent) where previous measurements of the intermittency factor indicated that transition had not begun. Boundary layer thinning in downwash zones could account for much of the heat transfer enhancement. The phenomenon could be a contributory factor to the long, apparently transitional regions often reported on blade cascade pressure surfaces, although the Gortler numbers where enhancement occurred are higher than those normally associated with pressure-surface transition.

Combined effects of rib angle-of-attack and pitch-to-height ratio on mass transfer from a surface with transverse ribs

Abstract: An experimental study is performed to investigate the effects of rib angle-of-attack ($ ) and pitch-to-height ratio (pit) on the mass transfer from a rib-roughened surface. For a given Reynolds number, Re - 4 x 104 distributions of local mass transfer coefficient with p/ e = 5 and 10 and θ = 60° and 90° are obtained in great detail. By integrating the measured local data, the surface with p/ e = 5 and 6 90° is found to have the highest pitch-averaged mass transfer coefficient among the four cases studied. This is in sharp contrast to the conventional notion that a rib surface with p/ e - 10 always yields a higher heat transfer enhancement than with p/ e - 5. A change of 9 from 90° to 60° results in opposite effects on the average mass transfer for pie = 5 and 10. The $ variation also strongly affects the local mass transfer characteristics, as expected. Very high mass transfer coefficients exist immediately ahead of a rib corner. This is speculated to be caused by vortex helical motion and periodic bounda...

Heat transfer enhancement in fin‐plate heat exchangers by wing type vortex generators

Abstract: Heat transfer and fluid mechanical data were computed for laminar channel flows containing strong longitudinal vortex pairs. The strong vortices are generated by thin delta wings and delta winglet pairs of low aspect ratios and large angles of attack. These wings are attached to the channel walls. The data show that longitudinal vortices cause high local peaks in heat transfer and marked increases in the overall channel heat transfer. These increases occur over a wide region of channel wall, compared to the vortex generating wing area. The results are of special interest for compact heat exchangers. The heat transfer enhancement allows a considerable reduction in the heat transfer area which, in turn, reduces the manufacturing and operating costs.

Heat Transfer Near the Entrance to a Film Cooling Hole in a Gas Turbine Blade

Abstract: : Film cooling is a method used to prevent jet engine turbine blade failure due to overheating. It consists of bleeding relatively cool air from the engine's compressor stage and discharging it through small holes in the turbine blade surface. This air provides a protective, insulating film which keeps the blade surface temperature well below the destructively high temperature levels of the combustor gases. This thesis presents for the first time, detailed pictures of the convective heat transfer distribution on the wall of an internal turbine blade passage near the entrance to a film cooling hole. The physical situation was modelled at 100X geometric scale as flow extraction into a single circular hole from a two-dimensional, fully developed, turbulent channel flow. High resolution heat transfer measurements were made using a transient technique with liquid crystals as surface temperature indicators. During the experiments, the two-dimensional channel Reynolds number was held constant while the flow extraction rate was varied for each of four hole inclination angles. The main region of heat transfer enhancement was found to be downstream with local heat transfer levels up to 6.5 times the levels associated with turbulent channel flow. Additional experimental, analytical, and computational flow field studies showed that the enhancement was caused mainly by the removal of the upstream boundary layer and the formation of a new laminar boundary layer at the down- stream hole edge. This new boundary layer was also influenced by downwash from a vortex pair.

Regional Heat Transfer and Pressure Drop in Two-Pass and Three-Pass Flow Passages With 180-Degree Sharp Turns

Abstract: The heat transfer distributions for flow passing through a two-pass (one-turn) and a three-pass (two-turn) passages with 180-degree sharp turns are studied by using the analogous naphthalene mass transfer technique. Both passages have square cross-section and length-to-height ratio of 8. The passage surface, including top wall, side walls and partition walls, is divided into 26 segments for the two-pass passage and 40 segments for the three-pass passage. Mass transfer results are presented for each segment along with regional and overall averages. The very non-uniform mass transfer coefficients measured around a sharp 180-degree turn exhibit the effects of flow separation, reattachment and impingement, in addition to secondary flows. Results of the three-pass passage indicate that heat transfer characteristics around the second turn is virtually the same as that around the first turn. This may imply that, in a multiple-pass passage, heat transfer at the first turn has already reached the thermally developed (periodic) condition. Over the entire two-pass passage, the heat transfer enhancement induced by the single-turn is about 45% to 65% of the fully developed values in a straight channel. Such a heat transfer enhancement decreases with an increase in Reynolds number. In addition, overall heat transfer of the three-pass passage is approximately 15% higher than that of the two-pass one. This 15% increase appears to be Reynolds number independent. The pressure loss induced by the sharp turns is found to be very significant. Within the present testing range, the pressure loss coefficient for both passages varies significantly with the Reynolds number.Copyright © 1989 by ASME

Fluidized bed combustion heat transfer enhancement

Abstract: A process for enhancing heat transfer between combustion products formed in a fluidized bed combustion (FBC) reactor and the water-containing tubes surrounding the reactor wherein particles of an inert material, e.g., mineral calcium sulfate (gypsum), having a spheroidal shape ranging in size from about 50 to 1000 μm and having a Moh's hardness of between 2.0 and 4.0 are introduced into the reactor in the presence of a solid fuel and an oxygen-containing gas. Introduction of such particles into the FBC reactor increases the particle density within the reactor thereby increasing radiant and convective heat transfer between the circulating bed and the surrounding water cooled walls. The improved process allows more precise control of the operating temperature regime. Erosion of the reactor walls is also substantially reduced.

Comparative Performance of Rippled Fin Plate Fin and Tube Heat Exchangers

Abstract: Continuous rippled fins are preferred to interrupted fins in applications where fouling by fibrous matter or insects is a problem. The performance characteristic of three rippled fin heat exchangers have been measured in a thermal wind tunnel. The results of these measurements are reported and comparisons are made wtih published data on similar surfaces. The performance evaluation criteria used as the basis for the comparisons were those recommened by Shah (1978). The tested rippled fin surfaces were found to have a higher performance than a similar surface reported in Kay and London (1984). The heat transfer enhancement was found to be dependent upon the profile of the fin.

Liquid Metal Turbulent Flow Phenomena and Their Implications on Fusion Reactor Blanket Design

Abstract: The experimental results of heat transfer enhancement in forced liquid metal flows due to the application of a transverse magnetic field are reviewed. It is shown that in nonconducting channels the heat transfer enhancement is due to the creation, by an inverse energy transfer process, of a strong anisotropic turbulence. For this to happen it is necessary to use flow perturbing means (such as grids) which inject to the flow turbulent energy in the form of small scale vortices whose axes are parallel to the field direction. In conducting channels, on the other hand, the magnetic field excites the turbulence without the need for external flow perturbing means. If persisting to the M > 300, Re > 2 x 104 range arrived at the present experiments, this magnetic field enhanced anisotropic turbulence and heat transfer could significantly improve the design and performance of liquid metal cooled fusion reactors.

Steam condensation in a vertical corrugated duct

Abstract: An experimental investigation was made on the condensation of water steam in a vertical corrugated duct. The data have been correlated as follows Co=5.11Re−0.431 150 ≤ Re ≤ 350 Co = 0.034311Re−0.425350 ≤Re ≤l000 The vertical corrugated duct is constructed of two corrugated plates with corrugation inclination angles of β = 0 and β = 45° respectively (relative to the overall flow direction). The condensation heat transfer coefficient in the corrugated duct is more than two times higher than that of bulk condensation on a vertical plate. A physical model was proposed to explain the heat transfer enhancement. Attention was also paid to the effect of exit steam velocity on the heat transfer during partial condensation. It was demonstrated that the heat transfer in the corrugated duct was strongly affected even at a low exit velocity, which was different from the case of bulk condensation on a vertical plate. Experimental apparatuses and the method for examining their reliability are described in detail.

Quantitative evaluation of heat transfer enhancement due to radial power distribution during reflood phase of PWR-LOCA.

Abstract: Experimental studies using Slab Core Test Facility (SCTF) and Cylindrical Core Test Facility (CCTF) indicated that the degree of heat transfer enhancement due to the radial power distribution during the reflood phase of a PWR-LOCA was governed mainly by the radial power ratio itself and less dependent on the shape of radial power distribution within the maximum power ratio of 1.36. The experimental condition covering the wide ranges of the reflood phase and the scale of core radius from 1/4.6 to 1/1 had little effect on the two- dimensional heat transfer behavior. The heat transfer coefficient under nonuniform radial power distribution was expressed as a sum of the heat transfer coefficient obtained under a complete mixing condition and an additional value given by an empirical correlation based on the SCTF results. The temperature rise at the peak power rod calculated with this expression tended to be lower than that calculated with the complete mixing model used in a reflood analysis code REFLA. That is...

ミスト冷却熱交換器に関する研究 : 第6報,運転条件と機器形状の最適化

Abstract: In order to pursue a high performance mist-cooling heat exchanger, experimental and theoretical studies on mist cooling have been performed on a heated horizontal tube bundle in a vertical spray crossflow. The dependency of the heat transfer performance on the configuration of the heat exchanger has been clarified. A method for optimizing the heat exchanger design was presented.

ミスト冷却熱交換器に関する研究 : 第5報,管群効果と熱伝達について

Abstract: In order to pursue a high performance mist-cooling heat exchanger, an experimental study of mist cooling has been performed on a heated tube bundle in an air-water mist flow. The mechanism of heat transfer enhancement was clarified in relation to the effect of a tube bundle. The operating condition was optimized for the heat exchanger configuration including a surface structure of heated tubes.

Internally drained condenser for spacecraft thermal management

Abstract: This paper presents the results obtained to date in a program to develop a high heat flux condenser for use in two-phase spacecraft thermal management loops. The objective is to obtain a several fold increase in condensation heat transfer coefficient over those which can be achieved with shear-controlled or capillary-wick condensers. The internally drained condenser relies on shaped fins to develop a capillary pressure gradient over the surface of the fins and drive the condensate toward narrow drainage grooves separating the fins. The condensate then flows through a drainage network embedded in the condenser walls. Heat transfer coefficients of up to 8 W/sq cm C were measured in steam, providing a heat transfer enhancement ratio greater than a factor of 8. In the paper the proof-of-concept experiments are described and simplified models to predict the performance of the internally drained condenser are presented.

衝突板 近傍에 配列된 2次元 rod가 衝突噴流 熱傳達에 미치는 影響 (1) : 傳熱面 - rod間의 間隙效果

Abstract: The purpose of this study is augmentation of heat transfer without additional power in a rectangular impinging air jet. As a method of passive heat transfer augmentation in a two-dimensional air jet, heat transfer surface of flat plate with rods is used. This study, particularly in the wall jet rigeon, investigates the effect of the clearance between the flat plate and rod. Mechanism of heat transfer enhancement is investigated by measuring the local heat transfer coefficient. It is concluded that the superposition of the effects of flow accelerlation through the clearance between the flat plate and the rod, and reattachment of the flow was the cause of the excellent performance. The overall heat transfer rate of flat plate with rods is about 1.5 times larger than that of flat plate without rods.