Heat Transfer Enhancement by EHD-Induced Oscillatory Flows

2006 ◽  
Vol 128 (9) ◽  
pp. 861-869 ◽  
Author(s):  
F. C. Lai ◽  
J. Mathew
Keyword(s):  
Heat Transfer ◽  
Electric Field ◽  
Heat Transfer Enhancement ◽  
Numerical Solutions ◽  
Reynolds Numbers ◽  
Secondary Flows ◽  
Temperature Fields ◽  
Transfer Enhancement ◽  
Periodic Flows ◽  
Small Reynolds Numbers

Prior numerical solutions of electrohydrodynamic (EHD) gas flows in a horizontal channel with a positive-corona discharged wire have revealed the existence of steady-periodic flows. It is speculated that heat transfer by forced convection may be greatly enhanced by taking advantage of this oscillatory flow phenomenon induced by electric field. To verify this speculation, computations have been performed for flows with Reynolds numbers varying from 0 to 4800 and the dimensionless EHD number (which signifies the effect of electric field) ranging from 0.05 to ∞. The results show that heat transfer enhancement increases with the applied voltage. For a given electric field, oscillation in the flow and temperature fields occurs at small Reynolds numbers. Due to the presence of oscillatory secondary flows, there is a significant enhancement in heat transfer.

Heat Transfer Enhancement by EHD-Induced Oscillatory Flows

2005 ◽  
Author(s):  
F. C. Lai ◽  
J. Mathew
Keyword(s):  
Heat Transfer ◽  
Electric Field ◽  
Heat Transfer Enhancement ◽  
Numerical Solutions ◽  
Reynolds Numbers ◽  
Secondary Flows ◽  
Temperature Fields ◽  
Transfer Enhancement ◽  
Periodic Flows ◽  
Small Reynolds Numbers

Prior numerical solutions of electrohydrodynamic (EHD) gas flows in a horizontal channel with a positive-corona discharged wire have revealed the existence of steady-periodic flows. It is speculated that heat transfer by forced convection may be greatly enhanced by taking advantage of this oscillatory flow phenomenon induced by electric field. To verify this speculation, computations have been performed for flows with Reynolds numbers varying from 0 to 4800 and the dimensionless EHD number (which signifies the effect of electric field) ranging from 0.06 to ∞. The results show that heat transfer enhancement increases with the applied voltage. For a given electric field, oscillation in the flow and temperature fields occurs at small Reynolds numbers. Due to the presence of oscillatory secondary flows, there is a significant enhancement in heat transfer.

Influence of Crossflow-Induced Swirl and Impingement on Heat Transfer in an Internal Coolant Passage of a Turbine Airfoil

2000 ◽  
Vol 122 (3) ◽  
pp. 587-597 ◽  
Author(s):  
S. V. Ekkad ◽  
G. Pamula ◽  
S. Acharya
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Reynolds Numbers ◽  
Secondary Flows ◽  
Transfer Enhancement ◽  
Feed System ◽  
Nusselt Numbers ◽  
Flow Through ◽  
Transient Liquid Crystal Technique ◽  
Measured Pressure

Detailed heat transfer distributions are presented inside a two-pass coolant channel with crossflow-induced swirl and impingement. The impingement and passage crossflow are generated from one coolant passage to the adjoining coolant passage through a series of straight or angled holes along the dividing wall. The holes provide for the flow turning from one passage to another typically achieved in a conventional design by a 180-deg U-bend. The holes direct the flow laterally from one passage to another and generate different secondary flow patterns in the second pass. These secondary flows produce impingement and swirl and lead to higher heat transfer enhancement. Three different lateral hole configurations are tested for three Reynolds numbers (Re=10,000, 25,000, 50,000). The configurations were varied by angle of delivery and location on the divider wall. A transient liquid crystal technique is used to measure the detailed heat transfer coefficient distributions inside the passages. Results with the new crossflow feed system are compared with the results from the traditional 180-deg turn passage. Results show that the crossflow feed configurations produce significantly higher Nusselt numbers on the second pass walls without affecting the first pass heat transfer levels. The heat transfer enhancement is as high as seven to eight times greater than obtained in the second pass for a channel with a 180-deg turn. The increased measured pressure drop (rise in friction factor) caused by flow through the crossflow holes are compensated by the significant heat transfer enhancement obtained by the new configuration. [S0022-1481(00)03103-0]

Numerical Study of EHD-Enhanced Forced Convection Using Two-Way Coupling

2003 ◽  
Vol 125 (4) ◽  
pp. 760-764 ◽  
Author(s):  
M. Huang ◽  
F. C. Lai
Keyword(s):  
Heat Transfer ◽  
Electric Field ◽  
Heat Transfer Enhancement ◽  
Forced Convection ◽  
Electric Fields ◽  
Numerical Solutions ◽  
Numerical Study ◽  
Transfer Enhancement ◽  
Wide Range ◽  
The Difference

Numerical results are presented for heat transfer enhancement using electric field in forced convection in a horizontal channel. The main objective of the present study is to verify the assumption that is commonly used in the numerical study of this kind of problem, which assumes that the electric field can modify the flow field but not vice versa (i.e., the so-called one-way coupling). To this end, numerical solutions are obtained for a wide range of governing parameters (V0=10, 12.5, 15 and 17.5 kV as well as ui=0.0759 to 1.2144 m/s) using both one-way and two-way couplings. The results obtained, in terms of the flow, temperature, and electric fields as well as the heat transfer enhancement, are thoroughly examined. Since the difference in the results obtained by two approaches is insignificant, it is concluded that the assumption of one-way coupling is valid for the problem considered.

Electrode Arrangement Effect on Natural Convection in a Horizontal Enclosure

2009 ◽  
Author(s):  
R. Ghazi ◽  
M. S. Saidi ◽  
M. H. Saidi
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Temperature Field ◽  
Natural Convection ◽  
Flow Field ◽  
Electric Field ◽  
Heat Transfer Enhancement ◽  
Temperature Fields ◽  
Transfer Enhancement ◽  
Rayleigh Numbers

The heat transfer enhancement of natural convection, using electrohydrodynamic technique inside a horizontal enclosure heated from below, is studied numerically. The interactions between electric field, flow field, and temperature field are investigated by CFD methods. The flow and temperature fields are affected by voltage applied at the wire electrodes. For different voltages and number of electrodes, it is noticed that the Nusselt number increased in all cases and the best enhancement is obtained at lower Rayleigh numbers. It is also shown that increasing the number of electrodes doesn’t always cause an increase in the heat transfer enhancement. Actually, when the number of electrodes is equal to the number of Be´nard cells, the best heat transfer enhancement is obtained.

Numerical Study of EHD-Enhanced Forced Convection Using Two-Way Coupling

2001 ◽  
Author(s):  
M. Huang ◽  
F. C. Lai
Keyword(s):  
Heat Transfer ◽  
Electric Field ◽  
Heat Transfer Enhancement ◽  
Forced Convection ◽  
Electric Fields ◽  
Numerical Solutions ◽  
Numerical Study ◽  
Transfer Enhancement ◽  
Wide Range ◽  
The One

Abstract In this paper, numerical results are presented for heat transfer enhancement using electric field in forced convection in a horizontal channel. The electric field is generated by charging a wire electrode located at the center of the channel with direct current at a high voltage. The main objective of the present study is to verify the assumption that is commonly used in the numerical study of this kind of problems, which assumes the electric field can modify the flow field but not vice versa (i.e., the so-called one-way coupling). To this end, numerical solutions have been obtained for a wide range of governing parameters (Vo = 10, 12.5, 15 and 17.5 kV as well as ui = 0.0759 to 1.2144 m/s) using both one-way and two-way couplings. Using the two-way coupling approach, the possible modification of the electric field by the primary flow, which was previously neglected, is accounted for. The results obtained using these two approaches, in terms of the flow, temperature, and electric fields as well as the heat transfer enhancement, are thoroughly examined. In addition, their influence over the flow stability is investigated. Finally, the conclusion about the validity of the one-way coupling is reached at the end of the study.

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.

Heat Transfer Enhancement in a Rectangular (AR = 3:1) Channel With V-Shaped Dimples

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
C. Neil Jordan ◽  
Lesley M. Wright
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Liquid Crystal ◽  
Pressure Drop ◽  
Heat Transfer Enhancement ◽  
Thermal Performance ◽  
Secondary Flows ◽  
Transfer Enhancement ◽  
Reduced Pressure

An alternative to ribs for internal heat transfer enhancement of gas turbine airfoils is dimpled depressions. Relative to ribs, dimples incur a reduced pressure drop, which can increase the overall thermal performance of the channel. This experimental investigation measures detailed Nusselt number ratio distributions obtained from an array of V-shaped dimples (δ/D = 0.30). Although the V-shaped dimple array is derived from a traditional hemispherical dimple array, the V-shaped dimples are arranged in an in-line pattern. The resulting spacing of the V-shaped dimples is 3.2D in both the streamwise and spanwise directions. A single wide wall of a rectangular channel (AR = 3:1) is lined with V-shaped dimples. The channel Reynolds number ranges from 10,000–40,000. Detailed Nusselt number ratios are obtained using both a transient liquid crystal technique and a newly developed transient temperature sensitive paint (TSP) technique. Therefore, the TSP technique is not only validated against a baseline geometry (smooth channel), but it is also validated against a more established technique. Measurements indicate that the proposed V-shaped dimple design is a promising alternative to traditional ribs or hemispherical dimples. At lower Reynolds numbers, the V-shaped dimples display heat transfer and friction behavior similar to traditional dimples. However, as the Reynolds number increases to 30,000 and 40,000, secondary flows developed in the V-shaped concavities further enhance the heat transfer from the dimpled surface (similar to angled and V-shaped rib induced secondary flows). This additional enhancement is obtained with only a marginal increase in the pressure drop. Therefore, as the Reynolds number within the channel increases, the thermal performance also increases. While this trend has been confirmed with both the transient TSP and liquid crystal techniques, TSP is shown to have limited capabilities when acquiring highly resolved detailed heat transfer coefficient distributions.

Heat Transfer in Internal Flows of Non-Linear Fluids: A Review

2006 ◽  
Author(s):  
Dennis A. Siginer ◽  
Mario F. Letelier
Keyword(s):  
Heat Transfer ◽  
Laminar Flow ◽  
Heat Transfer Enhancement ◽  
Polymer Solutions ◽  
Single Mode ◽  
Secondary Flows ◽  
Transfer Enhancement ◽  
Experimental Heat ◽  
Aqueous Polymer ◽  
Non Linear

A survey of the developments in heat transfer studies of non-linear inelastic as well as elastic fluids in tubes is given. Experimental findings concerning heat transfer enhancement characteristics of viscoelastic aqueous polymer solutions are very significant. Specifically, it is reported that heat transfer results for viscoelastic aqueous polymer solutions are drastically higher than those found for water in laminar flow in rectangular ducts. A number of investigators suggested that the high experimental heat transfer values were due to secondary flows resulting from the elasticity of the fluids. In this context recent results concerning the fully developed thermal field in constant pressure gradient driven laminar flow of a class of viscoelastic fluids characterized by single mode, non-affine constitutive equations in straight pipes of arbitrary contour ∂D is reviewed. Heat transfer enhancement due to shear-thinning is identified together with the enhancement due to the inherent elasticity of the fluid. The latter is the result of secondary flows in the cross-section. Increasingly large enhancements are computed with increasing elasticity of the fluid as compared to its Newtonian counterpart. Large enhancements are possible even with dilute fluids. Isotherms for the temperature field are presented and discussed for several non-circular contours such as the ellipse and the equilateral triangle together with heat transfer behavior in terms of the Nusselt number Nu.

Heat Transfer Enhancement in Electronic Modules Using Various Secondary Air Injection Hole Arrangements

1998 ◽  
Vol 120 (2) ◽  
pp. 342-347 ◽  
Author(s):  
B. A. Jubran ◽  
M. S. Al-Haroun
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Heat Transfer Enhancement ◽  
Reynolds Numbers ◽  
Air Injection ◽  
Transfer Enhancement ◽  
Injection Hole ◽  
Secondary Air Injection ◽  
Secondary Air ◽  
Compound Angle

This paper reports an experimental investigation to study the effects of using various designs of secondary air injection hole arrangements on the heat transfer coefficient and the pressure drop characteristics of an array of rectangular modules at different values of free-stream Reynolds numbers in the range 8 × 103 to 2 × 104. The arrangement used is either one staggered row of simple holes or one row of compound injection holes. The pitch distances between the injection holes, as well as the injection angles, were varied in both the streamwise and spanwise directions. Generally, the presence of secondary air through the injection hole arrangement can give up to 54 percent heat transfer enhancement just downstream of the injection holes. The amount of heat transfer enhancement and pressure drop across the electronic modules is very much dependent on the design of the injection holes. The simple angle injection hole arrangement tends to give a better heat transfer enhancement and less pressure drop than the compound angle holes.

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