The Spacer Grid Effect on Heat Transfer at Low Flow Rate in a 5×5 Rod Bundles

2016 ◽  
Author(s):  
Da Liu ◽  
Hanyang Gu ◽  
Shengjie Gong
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Enhancement ◽  
Flow Rate ◽  
Reynolds Numbers ◽  
Low Flow ◽  
Transfer Enhancement ◽  
Spacer Grid ◽  
Maximum Enhancement ◽  
Low Reynolds Numbers

It is widely acknowledged that the spacer grid has great effect on heat transfer downstream of it. The conventional correlations to predict the augmentation of the spacer were carried out on high Reynolds numbers. However, recent studies have shown that Reynolds number on the heat transfer enhancement is not negligible when the Reynolds number is lower than about 10000. An experiment to investigate the single-phase convective heat transfer downstream of the spacer grid at low flow rate has been performed in a 5×5 rod bundle. The test section was uniformly heated by a DC power and cooled by water. The Reynolds number covered from about 2000 to 10000. The experiment showed that the existing correlations for heat transfer enhancement by a spacer grid underestimated the maximum enhancement at the grid exit of the spacer grid at low Reynolds numbers. As the Reynolds number decreases, the maximum enhancement increases, nevertheless, when Reynolds number decreases to about 4300, the maximum enhancement tend to converge at a certain value. A new correlation has been proposed to account for the Reynolds number effect on heat transfer enhancement downstream of the spacer grid at low Reynolds numbers and which gave good predictions.

Air-Side Heat Transfer Enhancement and Pumping Power Penalty in Air-Cooled Heat Exchangers With Dimpled Fins

2017 ◽  
Author(s):  
Tung X. Vu ◽  
Lokanath Mohanta ◽  
Vijay K. Dhir
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Enhancement ◽  
Heat Exchangers ◽  
Fold Increase ◽  
Reynolds Numbers ◽  
Pumping Power ◽  
Transfer Enhancement ◽  
Power Penalty ◽  
Flat Tubes

In this work, we focus exclusively on heat transfer enhancement techniques for the air-side heat transfer in air-cooled heat exchangers/condensers. An innovative dimpled fin configuration is explored. Experiments, in which both heat transfer and drag are measured, are conducted with flat tubes in three configurations: without fins, with plain fins and with dimpled fins. Reynolds numbers based on the hydraulic diameter of the finned passages are varied between 600 and 7000. Results indicate that fins are more advantageous at lower Reynolds numbers since the increase in drag at higher Reynolds numbers quickly erases any advantage due to an increase in heat transfer rate. As an example, for the plain fins versus a bare tube at a Reynolds number of 600, there is a 7 fold increase in heat transfer with only a 5 fold increase in drag. However, at a Reynolds number of 7000, both heat transfer and drag increase by approximately 6 times, indicating that the increase in drag has caught up with the heat transfer enhancement. Similarly, while dimpled fins do result in higher heat transfer compared with the plain fins, the advantage is also more prominent at lower Reynolds numbers where heat transfer enhancement is higher than the associated increase in pumping power.

Numerical Investigation of Heat Transfer Enhancement Due to Flow Agitators Between Fins

2021 ◽  
pp. 1-14
Author(s):  
Aditya Patki ◽  
Shankar Krishnan
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Flow Visualization ◽  
Heat Transfer Enhancement ◽  
Time Scales ◽  
Reynolds Numbers ◽  
Critical Parameters ◽  
Transfer Enhancement ◽  
Oscillation Cycle

Abstract The paper investigates the heat transfer characteristics of a channel system consisting of mean axial flow and oscillatory cross flow components. A numerical model has been developed to solve the governing equations associated with the flow. The paper identifies advection, diffusion, and oscillation time scales and intensity of squeezing in the channel as critical parameters controlling system behavior. The total Reynolds number parameter is considered in the paper to understand the combined effect of axial and transverse Reynolds numbers on the Nusselt number. Flow visualization techniques are employed to understand the boundary layer changes that occur over an oscillation cycle. Nusselt number is found to increase with a reduction in advection and oscillation time scales. A linear relationship is observed between the Nusselt number and total Reynolds number when the axial and transverse Reynolds numbers are comparable. Non-dimensional pressure drop is primarily defined by only two parameters: axial Reynolds number and squeezing fraction. The flow visualization results indicate significant heat transfer enhancement in a small fraction of the oscillation cycle characterized by flow conditions similar to Couette flow.

Numerical Analysis of Thermofluid Performance of Fin-and-Tube Heat Transfer Surface Using Rectangular Winglets

2019 ◽  
Vol 141 (10) ◽  
Author(s):  
Shailesh Kumar Sarangi ◽  
Dipti Prasad Mishra ◽  
Praveen Mishra
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Heat Transfer Enhancement ◽  
Enhancement Factor ◽  
Reynolds Numbers ◽  
Heat Transfer Surface ◽  
Major Influence ◽  
Transfer Enhancement ◽  
Maximum Enhancement ◽  
Streamwise Distance

AbstractThis paper numerically investigates the heat transfer enhancement using rectangular winglet pairs in a fin-and-tube type heat transfer surface having five inline rows of tubes. The influence of number of winglets, attack angles of the winglets, and their location has been analyzed under laminar flow conditions with Reynolds number ranging 400–1500. To account for the combined effect of heat transfer enhancement and pressure drop penalty, an enhancement factor is also discussed by changing the winglet pair's number and location. The numerical results show that pressure drop can be reduced significantly by placing the winglet more toward the exit of the flow channel. Streamwise distance and spanwise distance of the winglet pairs have been investigated for maximum enhancement factor. The numerically obtained results show that the winglets number and their placement at different locations have a major influence on enhancement factor. The results show that both the heat transfer and the pressure drop increase with an increase in attack angle of the winglets and best angle for the highest enhancement factor has been found out. Correlations have been developed for streamwise distance, spanwise distance, and angle of attack for different range of Reynolds numbers.

Quantitative Experimental Investigation on the Flow Characteristics of Nanofluids in Turbulent Flow

2020 ◽  
Vol 142 (3) ◽  
Author(s):  
Jizu Lv ◽  
Chengzhi Hu ◽  
Zhenxian Zhang ◽  
Minli Bai
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Flow Direction ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Low Flow ◽  
Transfer Enhancement ◽  
Basic Fluid ◽  
Field Disturbance ◽  
Perturbation Effect

Abstract In addition to the increase of thermal conductivity, heat transfer enhancement mechanism for nanofluids also includes the changes of the flow characteristics, therefore it is needed to take an in-depth research on nanofluids flow characteristics. In this paper, the flow characteristics of H2O and SiO2-H2O nanofluids in a rectangular convex channel (channel composed of continuous staggered rectangular convex platform) at the Reynolds numbers 2300, 2500, 3000 and 4000 are studied by the quantitative PIV method (Fig. 1a). The rectangular convex channel (Fig. 1b) has periodic perturbation effect on the fluid flow, so that the flow direction is changed for several times, and vortexes are generated, which makes turbulence enhanced. In this way, flow is in the intense turbulent state under a low flow rate. Results show that the flow fields becomes more chaotic by the addition of nanoparticles (Fig. 2 and 3). Both the number and the size of vortices increase observably. The vorticity of nanofluids is also enhanced compared with H2O, and with the increase of Reynolds number, the increased ratio in the vorticity magnitude is getting higher (Fig. 4). At different Reynolds number, the pressure loss of nanofluids increases by 2.27%, 2.23%, 1.5% and 14.7%, respectively. As shown the flow resistance does not increase significantly compared to base fluids, especially at low Reynolds number. It can be concluded that the interaction between nanoparticles and the basic fluid strengthens the flow field disturbance, which is benefit to the heat transfer of nanofluids.

Using Computational Fluid Dynamics to Design Heat Transfer Enhancement Methods for Cooling Channels

1995 ◽  
Author(s):  
Aaron M. Plotnik ◽  
Ann M. Anderson
Keyword(s):  
Heat Transfer ◽  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Heat Transfer Enhancement ◽  
Reynolds Numbers ◽  
Pumping Power ◽  
Transfer Enhancement ◽  
Low Reynolds Numbers ◽  
Cooling Channels ◽  
Dimensional Measure

Abstract This paper presents the results of a study to enhance heat transfer in a short narrow cooling channel (7.9 mm high by 30.9 mm wide by 109 mm long). The study was performed using a computational fluid dynamics (CFD) code. Flotherm, by Flomerics, Inc. The work emphasizes the usefulness of CFD in the design process. The heat transfer enhancement was accomplished by placing thin rib-like protrusions in the channel. Simulations were run for two protrusion spacings, with a range of protrusion heights from 0.5 to 1.5 mm and a range of channel Reynolds number from 500 to 40,000. The results of a grid dependence study are presented and baseline comparisons are made to validate the computational model. The results show increases in channel Nusselt number of 10–160% while the friction factor increases by 10–5200%. The different configurations are compared using a non-dimensional measure of the pumping power and this shows that devices are most effective at low Reynolds numbers. The enhancement in heat transfer, the increase in friction loss and the worth in terms of pumping power would all have to be weighed with respect to the needs of a particular application before any choice is made to apply the techniques studied in this report.

Heat Transfer Investigation of the Channels With Rib Turbulators and Double-Row Bleed Holes

2011 ◽  
Author(s):  
Tao Guo ◽  
Huiren Zhu ◽  
Dunchun Xu
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Liquid Crystal ◽  
Heat Transfer Enhancement ◽  
Reynolds Numbers ◽  
Transfer Enhancement ◽  
Double Row ◽  
Average Heat ◽  
Rib Turbulators ◽  
Transient Liquid Crystal Technique

The detailed heat transfer distributions are measured for the wall of a channel with rib turbulators and double-row bleed holes by transient liquid crystal technique. The effects of the relative positions of rib turbulators and bleed holes, rib angles, channel Reynolds numbers and bleed ratios on heat transfer character are studied. The bleed holes are located near the upstream ribs, equidistant between ribs and near the downstream ribs. Three different rib angles of 60°, 90° and 120° are selected with the holes equidistant between ribs. The channel Reynolds numbers are changed from 30000 to 120000. The bleed ratios are between 0.09 and 0.22. The results show that angled ribs produces higher heat transfer enhancement in conjunction with the effect of bleed holes. The heat transfer characters are best when the bleed holes are located near the upstream ribs in the channels with 90° ribs. The change of bleed holes locations does not change the position of the flow reattachment, but affect the heat transfer distribution and intensity in the region. The average heat transfer enhancement decreases with the increasing of Reynolds number, and slight increases as the bleed ratio increases.

An Experimental and Numerical Study of Heat Transfer and Pressure Loss in a Rectangular Channel With V-Shaped Ribs

2006 ◽  
Author(s):  
Michael Maurer ◽  
Jens von Wolfersdorf ◽  
Michael Gritsch
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Thermal Performance ◽  
Pressure Loss ◽  
Numerical Study ◽  
Rectangular Channel ◽  
Reynolds Numbers ◽  
Transfer Enhancement ◽  
High Reynolds Numbers ◽  
High Reynolds

An experimental and numerical study was conducted to determine the thermal performance of V-shaped ribs in a rectangular channel with an aspect ratio of 2:1. Local heat transfer coefficients were measured using the steady state thermochromic liquid crystal technique. Periodic pressure losses were obtained with pressure taps along the smooth channel sidewall. Reynolds numbers from 95,000 to 500,000 were investigated with V-shaped ribs located on one side or on both sides of the test channel. The rib height-to-hydraulic diameter ratios (e/Dh) were 0.0625 and 0.02, and the rib pitch-to-height ratio (P/e) was 10. In addition, all test cases were investigated numerically. The commercial software FLUENT™ was used with a two-layer k-ε turbulence model. Numerically and experimentally obtained data were compared. It was determined that the heat transfer enhancement based on the heat transfer of a smooth wall levels off for Reynolds numbers over 200,000. The introduction of a second ribbed sidewall slightly increased the heat transfer enhancement whereas the pressure penalty was approximately doubled. Diminishing the rib height at high Reynolds numbers had the disadvantage of a slightly decreased heat transfer enhancement, but benefits in a significantly reduced pressure loss. At high Reynolds numbers small-scale ribs in a one-sided ribbed channel were shown to have the best thermal performance.

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