Hydraulic Diameter Ratio Research Articles

Flow and Heat Transfer Prediction in Rotating Rectangular Pin-Fin Channels (AR = 10:1)

Computations were performed to study the three-dimensional turbulent flow and heat transfer in a rotating narrow rectangular channel with staggered arrays of pin fins. The channel aspect ratio is 10:1, the pin length to diameter ratio is 1.0, and the pin spacing to hydraulic diameter ratio is 3.0 in both the streamwise (S L /D h ) and spanwise S T /D h ) directions. Various combinations of rotation numbers and coolant-to-wall density ratios were examined. A total of seven calculations have been performed with various rotation numbers and inlet coolant-to-wall density ratios. The rotation number and density ratio varied from 0.0 to 0.14 and from 0.1 to 0.40, respectively. The Reynolds number is fixed to 10,000. A finite volume code, FLUENT is used to predict the flow and heat transfer. The Reynolds stress model in conjunction with a two-layer model is used to compute the turbulent flow and heat transfer in the rotating channel. The computational results are in good agreement with experimental data.

Analysis of the heat transfer and friction in a ribbed square channel using numerical and experimental methods

Numerical predictions and experiment of a hydrodynamic and thermally developed turbulent flow through square channels with one or two ribbed walls were performed to determine the pressure drop and heat transfer. The CFX (version 5.7) software package was used for the computations. The rough wall had 45°-inclined square ribs. All four walls in the channel were heated, and a uniform heat flux was maintained on the entire inner heat transfer channel area. Experimental data were also obtained for four Reynolds numbers ranging from 7600 to 24 900, a pitch-to-rib-height ratio of 8.0, and a rib-height-to-channel hydraulic diameter ratio of 0.0667. The numerical results were in agreement with the experimental data and showed that the values of the local heat transfer coefficient and friction factor in a square channel with two ribbed walls were greater than those with one ribbed wall.

Flow and Heat Transfer Characteristics in Rectangular Channels With Staggered Transverse Ribs on Two Opposite Walls

The flow characteristics and the heat transfer properties of the rectangular channels with staggered transverse ribs on two opposite walls are experimentally studied. The rib height to channel height ratio ranges from 0.15 to 0.61 (rib height to channel hydraulic diameter ratio from 0.09 to 0.38). The pitch to rib height ratio covers from 2.5 to 26. The aspect ratio of the rectangular channel is 4. The flow characteristics are studied in a water channel, while the heat transfer experiments are performed in a wind tunnel. Particle image velocimetry (PIV) is employed to obtain the quantitative flow field characteristics. Fine-wire thermocouples imbedded near the inner surface of the bottom channel wall are used to measure the temperature distributions of the wall and to calculate the local and average Nusselt numbers. Using the PIV measured streamline patterns, various characteristic flow modes, thru flow, oscillating flow, and cell flow, are identified in different regimes of the domain of the rib height to channel height ratio and pitch to rib height ratio. The vorticity, turbulence intensity, and wall shear stress of the cell flow are found to be particularly larger than those of other characteristic flow modes. The measured local and average Nusselt numbers of the cell flow are also particularly higher than those of other characteristic flow modes. The distinctive flow properties are responsible for the drastic increase of the heat transfer due to the enhancement of the momentum, heat, and mass exchanges within the flow field induced by the large values of the vorticity and turbulence intensity. Although the thru flow mode is conventionally used in the ribbed channel for industrial application, the cell flow could become the choice if the heat transfer rate, instead of the pressure loss, is the primary concern.

Experimental investigation of local heat transfer in a square duct with various-shaped ribs

In the present study, experimental studies are carried out to investigate the heat transfer and friction characteristics in a square duct roughened by various-shaped ribs on one wall. The ribs are oriented transversely to the main stream in a periodic arrangement. Liquid crystal thermography is employed to measure the local and average heat transfer coefficient on the ribbed surface. The rib height-to-duct hydraulic diameter ratio is fixed at 0.1; the rib pitch-to-height ratio varies from 8 to 15 and the test Reynolds number spans from 8,000 to 20,000. The results show that the trapezoidal-shaped ribs with decreasing height in the flow direction (case C) provide the highest heat transfer enhancement factor and are likely to be used to suppress the local hot spot which usually occurs in the region just behind the ribs.

Research on diffusion in micro-channel flow driven by electroosmosis

Numerical simulation using the finite differential method was carried out to analyze the diffusion of an impulse sample in the micro-channel driven by electroosmosis. The results show that the electrical field strength applied externally and the concentration of buffer solution play a significant role in the diffusion of sample, however, hydraulic diameter and aspect ratio of height to width of channel play a small role in it. Weakening the electrical field strength applied externally and the concentration of buffer solution properly can prevent the sample band from broadening effectively, and promote the efficiency of testing and separation as well as keep a faster speed of transport. The conclusions are helpful to the optimal design for micro-channel.

Estimation of Joule heating effect on temperature and pressure distribution in electrokinetic‐driven microchannel flows

In this study we present simple analytical models that predict the temperature and pressure variations in electrokinetic-driven microchannel flow under the Joule heating effect. For temperature prediction, a simple model shows that the temperature is related to the Joule heating parameter, autothermal Joule heating parameter, external cooling parameter, Peclet number, and the channel length to channel hydraulic diameter ratio. The simple model overpredicted the thermally developed temperature compared with the full numerical simulation, but in good agreement with the experimental measurements. The factors that affect the external cooling parameters, such as the heat transfer coefficient, channel configuration, and channel material are also examined based on this simple model. Based on the mass conservation, a simple model is developed that predicts the pressure variations, including the temperature effect. An adverse pressure gradient is required to satisfy the mass conservation requirement. The temperature effect on the pressure gradient is via the temperature-dependent fluid viscosity and electroosmotic velocity.

Evaluating the role of subgrid stress modeling in a ribbed duct for the internal cooling of turbine blades

Time-dependent simulations are performed in a ribbed square duct with rib height to hydraulic diameter ratio of 0.1 and rib pitch to rib height ratio of 10. The calculations are performed for a nominal bulk Reynolds number of 20,000. Hydrodynamic and thermal fully-developed conditions are assumed. Two mesh resolutions, 96 3 and 128 3 are tested in a quasi-DNS mode and in LES mode with the Dynamic Smagorinsky model. Time evolution, mean, and turbulent quantities are presented, together with friction and heat transfer. It is found that in general, both quasi-DNS and LES resolve the bulk mean features of the flow within 10–15% of each other. These include recirculation patterns and secondary flows which are characteristic of this geometry. However, there are large differences in predicting the friction and heat transfer coefficients, both of which are very sensitive to the predicted turbulent field near the duct surfaces. Both quasi-DNS calculations underpredict the heat transfer and friction coefficient by amounts which range between 20% and 30% on the 96 3 mesh and 15–20% on the 128 3 mesh. However, the LES calculations with the dynamic Smagorinsky model predict these quantities within 5–10% of experimental values for both mesh resolutions. It is concluded that in an average sense, the level of turbulence augmentation provided by the dynamic model is commensurate with the mesh resolution such that the turbulent energy, heat transfer coefficient, and friction are predicted at the right levels independent of the resolution.

Liquid crystal heat transfer measurements in a rectangular channel with solid and slit rib

An experimental investigation on the heat transfer effectiveness of solid and slit ribs mounted on the bottom surface of a rectangular channel has been carried out at Reynolds numbers of 13400, 22600, 32100 and 40800. The rib height to hydraulic diameter ratio (e/D_h) set during experiment is equal to 0.0624. The surface Nusselt number results from transient liquid crystal thermography are presented. The heat transfer enhancement performance analysis has been carried out using entropy generation principle. The slit rib is superior to solid rib from both heat transfer augmentation and pressure penalty point of view. The performance of the slit rib is a function of the open area ratio (β) and the location of the slit (b) from the bottom test surface. The optimum open area ratio is 20% and the slit located symmetrically from the top and bottom surface of the rib is the optimum location of the slit. The heat transfer augmentation of the slit rib (β = 20%) is 61% in comparison to 40% for the solid rib at Re = 32100 and the pressure penalty for the slit rib is 7% lower than the solid rib. The entropy generation for the slit rib is 33% less than that of the solid rib.

Use of porous baffles to enhance heat transfer in a rectangular channel

An experimental investigation was carried out to measure module average heat transfer coefficients in uniformly heated rectangular channel with wall mounted porous baffles. Baffles were mounted alternatively on top and bottom of the walls. Heat transfer coefficients and pressure loss for periodically fully developed flow and heat transfer were obtained for different types of porous medium (10, 20, and 40 pores per inch (PPI)) with two window cut ratios ( B h/ D h=1/3 and 2/3) and two baffle thickness to channel hydraulic diameter ratios ( B t/ D h=1/3 and 1/12). Reynolds number ( Re) was varied from 20,000 to 50,000. To compare the effect of foam metal baffle, the data for conventional solid-type baffle were obtained for ( B t/ D h=1/3). The maximum uncertainties associated with module Nusselt number and friction factor were 5.8% and 4.3% respectively. The experimental procedure was validated by comparing the data for the straight channel with no baffles ( B h/ D h=0) with those in the literature [Publications in Engineering, vol. 2, University of California, Berkeley, 1930, p. 443; Int. Chem. Eng. 16 (1976) 359]. The use of porous baffles resulted in heat transfer enhancement as high as 300% compared to heat transfer in straight channel with no baffles. However, the heat transfer enhancement per unit increase in pumping power was less than one for the range of parameters studied in this work. Correlation equations were developed for heat transfer enhancement ratio and heat transfer enhancement per unit increase in pumping power in terms of Reynolds number.

Heat Transfer in Two-Pass Rotating Rectangular Channels (AR=2) With Five Different Orientations of 45 Deg V-Shaped Rib Turbulators

An experimental study was made to obtain heat transfer data for a two-pass rectangular channel (aspect ratio=2:1) with smooth and ribbed surfaces for two channel orientations (90 deg and 135 deg with respect to the plane of rotation). The V-shaped ribs are placed on the leading and trailing surfaces. Five different arrangements of 45 deg V-shaped ribs are studied. The Reynolds number and rotation number ranges are 5000–40000, and 0.0–0.21, respectively. The rib height to hydraulic diameter ratio (e/D) is 0.094; the rib pitch-to-height ratio (P/e) is 10; and the inlet coolant-to-wall density ratio (Δρ/ρ) is maintained around 0.115 for every test. The results show that the rotation-induced secondary flow enhances the heat transfer of the first pass trailing surface and second pass leading surface. However, the first pass leading and the second pass trailing surfaces show a decrease in heat transfer with rotation. The results also show that parallel 45 deg V-shaped rib arrangements produce better heat transfer augmentation than inverted 45 deg V-shaped ribs and crossed 45 deg V-shaped ribs, and a 90 deg channel orientation produces greater rotating effect on heat transfer than a 135 deg orientation.

Heat transfer and friction behaviors in rectangular channels with varying number of ribbed walls

An experimental study of surface heat transfer and friction characteristics of a fully developed turbulent air flow in a square channel with transverse ribs on one, two, three, and four walls is reported. Tests were performed for Reynolds numbers ranging from 10,000 to 80,000. The pitch-to-rib height ratio, P/ e, was kept at 8 and rib-height-to-channel hydraulic diameter ratio, e/ D h was kept at 0.0625. The channel length-to-hydraulic diameter ratio, L/ D h, was 20. The heat transfer coefficient and friction factor results were enhanced with the increase in the number of ribbed walls. The friction roughness function, R( e +), was almost constant over the entire range of tests performed and was within comparable limits of the previously published data. The heat transfer roughness function, G( e +), increased with roughness Reynolds number and compared well with previous work in this area. Both correlations could be used to predict the friction factor and heat transfer coefficient in a rectangular channel with varying number of ribbed walls. The results of this investigation could be used in various applications of turbulent internal channel flows involving different number of rib roughened walls.

Heat Transfer in a Two-Pass Rectangular Rotating Channel With 45-deg Angled Rib Turbulators

Abstract Experimental heat transfer results are presented in a two-pass rectangular channel (aspect ratio=2:1) with smooth and ribbed surfaces for two channel orientations (90 and 135 deg to the direction of rotational plane). The rib turbulators are placed on the leading and trailing sides at an angle 45 deg to the main stream flow. Both 45-deg parallel and cross rib orientations are studied. The results are presented for stationary and rotating cases at three different Reynolds numbers of 5000, 10,000, and 25,000, the corresponding rotation numbers are 0.21, 0.11, and 0.04. The rib height to hydraulic diameter ratio (e/D) is 0.094; the rib pitch-to-height ratio (P/e) is 10 and the inlet wall-to-coolant density ratio (Δρ/ρ) is maintained at 0.115 for all surfaces in the channel. Results show that the rotating ribbed wall heat transfer coefficients increase by a factor of 2 to 3 over the rotating smooth wall results. The heat transfer from the first pass trailing and second pass leading surfaces are enhanced by rotation. However, the first pass leading and the second pass trailing sides show a decrease in heat transfer with rotation. The result show that 45-deg parallel ribs produce a better heat transfer augmentation than 45-deg cross ribs, and a 90-deg channel orientation produces higher heat transfer effect over a 135-deg orientation.

The Effect of Simultaneously Developing Flow on Heat Transfer in Rectangular Tubes

A study of heat transfer in simultaneously developing flow through rectangular tubes is presented in this article. Heat transfer coefficients were measured for three different tube sizes and shapes (D h = 2.21 mm, f = 0.050; D h = 3.02 mm, f = 0.108; and D h = 1.74 mm, f = 0.029), which correspond to typical dimensions used in automotive heat exchangers. For each of these tubes, several different tube lengths were tested to measure the effect of developing flow on the Nusselt number. The results demonstrate that developing flow enhances Nusselt numbers, especially for short tubes. For the geometry range studied, the effect of aspect ratio was not very significant. Heat transfer correlations that accounted for the effects of Reynolds number (118 < Re < 10,671) Prandtl number (6.48 < Pr < 16.20), and bulk-to-wall property variations (0.243 < w b / w w < 0.630), and geometric features such as tube length, hydraulic diameter, and aspect ratio, were developed from the data.

Flowfield Investigation of the Effect of Rib Open Area Ratio in a Rectangular Duct

The spatially periodic turbulent fluid flows and friction in a rectangular passage of width-to-height ratio of 4:1 with perforated rectangular ribs mounted on one wall have been studied using laser Doppler velocimetry and pressure probing. The parameters fixed were rib height to duct hydraulic diameter ratio of 0.106, rib width-to-height ratio of 0.76, rib pitch-to-height ratio of 10, and Reynolds number of 2 × 104, while the main parameter investigated was the rib open-area ratio (β) with values of 0%, 10%, 22%, 38%, and 44%. Two critical ranges of β and three characteristic flow regimes were identified, which provides useful references of practical tests of computational models. The results also showed that the dominant fluid dynamic factors responsible for the reported peak values of local Nusselt number distribution could be recognized. Moreover, the secondary-flow mean velocity components were found to be one to two orders of magnitude smaller than the bulk mean velocity.

Turbulent heat and fluid flow in a passage disturbed by detached perforated ribs of different heights

The spatially periodic turbulent heat transfer and friction in a rectangular passage of width-to-height ratio of 4:1 with perforated rectangular ribs detached from one wall have been studied using laser holographic interferometry, smoke-streak flow visualization, and pressure probing for rib-to-duct hydraulic diameter ratios ( H/De) of 0.081, 0.106, and 0.162 and Reynolds number ( Re) range of 5 × 10 3−5 × 10 4. The rib pitch-to-height ratio and the detached distance-to-rib-height ratio are 10 and 0.38, respectively. The laser-Doppler velocimetry measurements were performed to illustrate the local Nusselt number distribution. In contrast to the attached solid-type ribs which provide a monotonic decrease of thermal performance at a constant pumping power ( Nu p / Nu s ∗ ) with increasing rib height, the detached perforated-type ribs give a best Nu p / Nu s ∗ at a moderate rib height H/ De = 0.106 for the lower Re range and a Nu p / Nu s ∗ independent of H/De for the higher Re range. In addition, the thermal performance of the detached solid-type ribs is found to be superior to that of the detached perforated-type ribs and the physical reason for the difference is addressed.

The Effect of Periodic Ribs on the Local Aerodynamic and Heat Transfer Performance of a Straight Cooling Channel

The local aerodynamic and heat transfer performance were measured in a rib-roughened square duct as a function of the rib pitch to height ratio. The blockage ratio of these square obstacles was 10 or 20 percent depending on whether they were placed on one single (1s) or on two opposite walls (2s). The Reynolds number, based on the channel mean velocity and hydraulic diameter, was fixed at 30,000. The aerodynamic description of the flow field was based on local pressure distributions along the ribbed and adjacent smooth walls as well as on two-dimensional LDV explorations in the channel symmetry plane and in two planes parallel to the ribbed wall(s). Local heat transfer distributions were obtained on the floor, between the ribs, and on the adjacent smooth side wall. Averaged parameters, such as friction factor and averaged heat transfer enhancement factor, were calculated from the local results and compared to correlations given in literature. This contribution showed that simple correlations derived from the law of the wall similarity and from the Reynolds analogy could not be applied for the present rib height-to-channel hydraulic diameter ratio (e/Dh = 0.1). The strong secondary flows resulted in a three-dimensional flow field with high gradients in the local heat transfer distributions on the smooth side walls.

Effect of Rib Profiles on Turbulent Channel Flow Heat Transfer

Introduction R EPEATED ribs have been used as the promoters of turbulence to enhance the heat transfer to the  ow of coolants in channels. Several publications have addressed the comprehensive review of turbine blade cooling and the analysis of heat transfer and friction characteristics of  ow in channels with two opposite rib-roughened walls. The effects of  ow Reynolds number and rib geometry (rib height e, rib spacing P, rib angle of attack a, and rib orientation) on heat transfer and pressure drop in the fully developed region of square, rectangular, and triangular channels have been investigated. 4 Semiempirical correlations over a wide range of rib geometry for the friction and heat transfer design calculations are derived from the law-of-the-wall similarity for  ow over rough surfaces. A detailed analysis of the application of the similarity laws in the case of rectangular channels is presented by Han. Also, studies of the effect of a number of channel-ribbed walls on heat and friction characteristics have been reported. Experiments with some nonrectangular rib shapes have been reported. The previously mentioned studies were limited in the number of rib shapes and were for different conditions (rib height to channel hydraulic diameter ratio e/Dh, and Reynolds number Re). This study focuses on the effect of different rib proŽ les on turbulent channel  ow heat transfer and friction characteristics.

Compressibility effect on the gas flow and heat transfer in a microtube

This study numerically investigated the effect of gas compressibility on the friction factor in rectangular microchannels. The numerical model adopted in the study was validated against experimental data obtained by testing rectangular microchannels with a hydraulic diameter of 295 μm. The numerical model was used to evaluate the Reynolds number at which the compressibility effects on the friction factor became significant by analyzing the role of both hydraulic diameter and aspect ratio of the microchannel. To this end, three values of the hydraulic diameter (100, 295, and 500 μm) and five different aspect ratios (from 0.25 to 1) were investigated. The results showed that compressibility effects became increasingly stronger by reducing the hydraulic diameter and that they led to an increase in the average friction factor. For smaller microchannels, the Reynolds numbers at which the compressibility effects became significant tended to reduce and were in the laminar regime. The gas compressibility could not be ignored when friction factors needed to be accurately determined. Moreover, for narrow microchannels (low aspect ratio), compressibility effects became important for higher values of the Reynolds number than those observed for nearly squared microchannels.

Heat Transfer and Friction in a Low‐Aspect‐RatioRectangular Channel with Staggered Slit‐Ribbed Walls

Fully developed heat transfer and friction in a rectangular channel with slit‐ribbed walls are examined experimentally. The slit ribs are transversely arranged on the bottom and top channel walls in a staggered manner. Effects of rib open‐area ratio (β= 24%, 37%, and 46%), rib pitch‐to‐height ratio (Pi/H = 10, 15and20), and Reynolds number (10, 000 ≤ Re≤50, 000) are examined. The rib height‐to‐channel hydraulic diameter ratio is fixed at H/De = 0.081. It is disclosed that the heat transfer coefficient for the slit‐ribbed channel is higher than that for the solid‐ribbed channel, and increases with rib open‐area ratio. Results also show that the friction factor for the slit‐ribbed channel is significantly lower than that for the solid‐ribbed one. Moreover, the ribs with larger open‐area ratios in a higher flow Reynolds number condition could give the better thermal performance under the constant friction power constraint. Roughness functions for friction and heat transfer are further developed in terms of rib and flow parameters.

Heat Transfer Augmentation in a Rectangular Channel With Slit Rib-Turbulators on Two Opposite Walls

The effect of slit ribs on heat transfer and friction in a rectangular channel is investigated experimentally. The slit ribs are arranged in-line on two opposite walls of the channel. Three rib open-area ratios (β = 24, 37, and 46 percent), three rib pitch-to-height ratios (Pi/H = 10, 20, and 30), and two rib height-to-channel hydraulic diameter ratios (H/De = 0.081, and 0.162) are examined. The Reynolds number ranges from 10,000 to 50,000. Laser holographic interferometry is employed to measure the local heat transfer coefficients of the ribbed wall quantitatively, and observe the flow over the ribbed wall qualitatively. The results show that the slit rib has an advantage of avoiding “hot spots.” In addition, the heat transfer performance of the slit-ribbed channel is much better than that of the solid-ribbed channel. Semi-empirical correlations for friction and heat transfer are developed to account for rib spacings and open-area ratios. These correlations may be used in the design of turbine blade cooling passages.