Natural Convective Heat Transfer From an Unconfined or a Confined Heated Surface

2005 ◽  
Author(s):  
L. K. Liu ◽  
T. W. Lin ◽  
M. C. Wu ◽  
C. H. Peng ◽  
Y. H. Hung
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Steady State ◽  
Local Nusselt Number ◽  
Average Nusselt Number ◽  
Heated Surface ◽  
Nusselt Numbers ◽  
Extended Surface ◽  
Extended Surfaces ◽  
State Average

A series of experimental investigations with a stringent measurement method on the natural heat transfer from an unconfined or confined smooth and extended surface have been successfully conducted. From the results, the maximum transient-/steady-state local Nusselt number exists in the region near the edge of the heated smooth or extended surface, and the transient-/ steady-state local Nusselt number decreases along the distance from the surface edge toward the surface center. The transient-/steady-state local and average Nusselt number increases with increasing Grs, H/W or Hes/W. The effects of Grs, H/W and Hes/W on the Nus/Nus,o distribution are not significant; and the Nus/Nus,o distribution can be expressed as a generalized bowl-shaped profile, which is independent of Grs, H/W and Hes/W. By the statistical sensitivity analysis of ANOVA F-test, the steady-state average Nusselt number for unconfined/confined smooth or extended surface is significantly affected by either one of Grs, H/W and Hes/W. Among them, Grs has the most significant effect. Four new correlations of steady-state average Nusselt number in terms of relevant influencing parameters for unconfined/ confined smooth or extended surfaces are proposed, respectively. Furthermore, two normalized steady-state average Nusselt numbers for confined smooth or extended surfaces are proposed, respectively. As compared with the steady-state average Nusselt numbers for unconfined/confined smooth surface, the steady-state heat transfer enhancement for unconfined/confined extended surface can be achieved between 93.99% and 254.65%.

Convective Heat Transfer From Confined Heated Surfaces With Slot Jet Impingement

2005 ◽  
Author(s):  
L. K. Liu ◽  
C. J. Fang ◽  
M. C. Wu ◽  
C. H. Peng ◽  
Y. H. Hung
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Steady State ◽  
Convective Heat ◽  
Local Nusselt Number ◽  
Jet Impingement ◽  
Experimental Investigations ◽  
Surface Center ◽  
Nusselt Numbers ◽  
Extended Surface

A series of experimental investigations with a stringent measurement method on the transient-/steady-state heat transfer behavior for confined smooth surfaces with slot jet impingement have been successfully conducted. From the results, a generalized correlation is proposed to represent the distributions of normalized transient convective heat flux. The highest heat transfer during the transient period occurs at the surface center of confined heated smooth or extended surface. The transient local Nusselt number decreases along the distance from the surface center toward the surface edge. The transient-/steady-state local and average Nusselt numbers are almost independent of Grs, and they are more significantly affected by ReD as compared with H/W. They will increase with increasing ReD. Maximum local and average Nusselt numbers can be found between H/W = 3 and H/W = 5. The effects of Grs and H/W on the dimensionless local Nusselt number distribution are insignificant; and the distribution can be expressed as a generalized bell-shaped profile, which is only dependent of ReD. Finally, a new composite correlation of steady-state average Nusselt number for mixed convection from confined smooth due to slot jet impingement and buoyancy are presented.

Forced Convective Heat Transfer for Partially-Confined Compact PPF Heat Sinks With Top-Bypass Effect

2005 ◽  
Author(s):  
T. Y. Wu ◽  
M. P. Wang ◽  
J. T. Horng ◽  
S. F. Chang ◽  
Y. H. Hung
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Steady State ◽  
Convective Heat ◽  
Average Nusselt Number ◽  
Heat Dissipation ◽  
Heat Sinks ◽  
Nusselt Numbers ◽  
Transient Period ◽  
State Average

A series of experimental investigations with a stringent measurement method on the study of the fluid flow and heat transfer for confined compact heat sinks in forced convection have been successfully conducted. From the results, the thermal capacity of the heat sink and the convective heat dissipation play the major roles for dominating the transient thermal behavior in the beginning of power-on transient period; while, the convective heat dissipation finally becomes the solely dominating term at the end of power-on transient period. The transient/steady-state local and average Nusselt numbers increase with increasing Grs, H/Hc ratio or Re. As compared with the steady-state average Nusselt number for non-compact heat sink (Fin-Al/ Base-Al), the steady-state heat transfer enhancement for compact heat sinks (Fin-Al/Base-Al) is 185.74%. Furthermore, a new correlation of steady-state average Nusselt number in terms of relevant influencing parameters for confined compact PPF heat sinks in forced convection is proposed. As compared with two existing correlations of steady-state average Nusselt numbers for unconfined and confined non-compact PPF heat sinks, the heat transfer enhancements for the present confined compact PPF heat sinks of H/Hc = 0.47 are 423.29% and 219.93%, respectively.

Convective Heat Transfer From Heated Extended Surfaces With a Confined Slot Jet Impingement

2004 ◽  
Author(s):  
L. K. Liu ◽  
M. C. Wu ◽  
C. J. Fang ◽  
Y. H. Hung
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Reynolds Number ◽  
Mixed Convection ◽  
Average Nusselt Number ◽  
Jet Impingement ◽  
Separation Distance ◽  
Nusselt Numbers ◽  
Transfer Behavior ◽  
Extended Surfaces

A series of experimental investigations with stringent measurement methods on the studies related to mixed convection from the horizontally confined extended surfaces with a slot jet impingement have been successfully conducted. The relevant parameters influencing mixed convection performance due to jet impingement and buoyancy include the Grashof number, ratio of jet separation distance to nozzle width, ratio of extended surfaces height to nozzle width and jet Reynolds number. The range of these parameters studied are Grs = 3.77 × 105 – 1.84 × 106, H/W = 1–10, Hs/W = 0.74–3.40 and Re = 63–1383. In the study, the heat transfer behavior on the extended surfaces with confined slot jet impingement such as the temperature distribution, local and average Nusselt numbers on the extended surfaces has been systematically explored. The results manifest that the effect of steady-state Grashof number on heat transfer behavior such as stagnation, local and average Nusselt number is not significant; while the heat transfer performance increases with decreasing jet separation distance or with increasing extended surface height and jet Reynolds number. Besides, two new correlations of local and average Nusselt numbers in terms of H/W, Hs/W and Re are proposed for the cases of extended surfaces. A satisfactory agreement is achieved between the results predicted by these correlations and the experimental data. Finally, a complete composite correlation of steady-state average Nusselt number for mixed convection due to jet impingement and buoyancy is proposed. The comparison of the predictions evaluated by this correlation with all the present experimental data is made. The maximum and average deviations of the predictions from the experimental data are 7.46% and 2.87%, respectively.

On the Transiant Response of Convective Extended Surface Heat Transfer: A New Approach

2006 ◽  
Author(s):  
P. Razelos ◽  
G. Michalakeas
Keyword(s):  
Heat Transfer ◽  
Steady State ◽  
Transient Response ◽  
Surface Heat ◽  
Constant Thickness ◽  
Conduction Model ◽  
Steady State Condition ◽  
Surface Heat Transfer ◽  
Extended Surface ◽  
Extended Surfaces

This work is devoted to the study of the extended surfaces transient response. Although, the steady-state fin analysis has attracted considerable attention for a very long time, the interest in the transient response started in the last quarter of the past century. Several publications have appeared since, either analytical using the 1-D, conduction model, or experimental. Perusing the pertinent literature, however, we have observed that, in all previous published papers the authors treat the transient response of extended surfaces, or fins, like regular solids. However, fin endeavors rest on certain fundamental concepts, leading to some simplified assumptions, that we shall briefly discuss in the next section, which allows using the 1-D conduction model, and affect their steady-state operation. Therefore, the need for re-examining and revising the previously used methods becomes apparent. However, the authors are indebted to the pioneer workers on this topic that opened new avenues in the field of extended surface heat transfer. The aim of this work is to offer a different point of view to this problem, by introducing a new spatial coordinate system, and a new time scale. The solutions presented here, rest on the previously mentioned certain fundamental concepts developed recently. In the following we show step by step, how the existing pertinent equations and formulas of fins' transient response, are transformed to new simpler forms, expressed in terms of more appropriate dimensionless parameters, in accord with those appearing in recent publications. In the following, we confine to the analysis of constant thickness longitudinal and pin fins subject to specific1 boundary conditions. Each case is accompanied with an example that, for reasons of comparison are taken from the literature. We also discuss what is meant by "the time required for transient response to attain the steady-state condition."

Experimental Investigation of Coil Curvature Effect on Heat Transfer and Pressure Drop Characteristics of Shell and Coil Heat Exchanger

2014 ◽  
Vol 7 (1) ◽  
Author(s):  
M. R. Salem ◽  
K. M. Elshazly ◽  
R. Y. Sakr ◽  
R. K. Ali
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Exchanger ◽  
Friction Factor ◽  
Heat Exchangers ◽  
Average Nusselt Number ◽  
Transfer Coefficients ◽  
Nusselt Numbers ◽  
Fanning Friction Factor ◽  
Coil Heat

The present work experimentally investigates the characteristics of convective heat transfer in horizontal shell and coil heat exchangers in addition to friction factor for fully developed flow through the helically coiled tube (HCT). The majority of previous studies were performed on HCTs with isothermal and isoflux boundary conditions or shell and coil heat exchangers with small ranges of HCT configurations and fluid operating conditions. Here, five heat exchangers of counter-flow configuration were constructed with different HCT-curvature ratios (δ) and tested at different mass flow rates and inlet temperatures of the two sides of the heat exchangers. Totally, 295 test runs were performed from which the HCT-side and shell-side heat transfer coefficients were calculated. Results showed that the average Nusselt numbers of the two sides of the heat exchangers and the overall heat transfer coefficients increased by increasing coil curvature ratio. The average increase in the average Nusselt number is of 160.3–80.6% for the HCT side and of 224.3–92.6% for the shell side when δ increases from 0.0392 to 0.1194 within the investigated ranges of different parameters. Also, for the same flow rate in both heat exchanger sides, the effect of coil pitch and number of turns with the same coil torsion and tube length is remarkable on shell average Nusselt number while it is insignificant on HCT-average Nusselt number. In addition, a significant increase of 33.2–7.7% is obtained in the HCT-Fanning friction factor (fc) when δ increases from 0.0392 to 0.1194. Correlations for the average Nusselt numbers for both heat exchanger sides and the HCT Fanning friction factor as a function of the investigated parameters are obtained.

Localized Heat Transfer in Buoyancy Driven Convection in Open Cavities

2006 ◽  
Vol 129 (2) ◽  
pp. 167-178 ◽  
Author(s):  
Wilson Terrell ◽  
Ty A. Newell
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Average Nusselt Number ◽  
Convection Heat Transfer ◽  
Ambient Air ◽  
Nusselt Numbers ◽  
Transient Nature ◽  
Inclination Angles ◽  
Buoyancy Driven Convection ◽  
The Heat Transfer Coefficient

Background. An experimental study of buoyancy driven convection heat transfer in an open cavity was conducted. Method of Approach. Test cavities were constructed with calorimeter plates bonded to Styrofoam insulation. The inside of the cavities was heated and then exposed to ambient air for approximately thirty minutes. Different size cavities were examined at inclination angles of 0, 45, and 90deg. The heat transfer coefficient was determined from an energy balance on each calorimeter plate. The cavity’s plate temperatures varied spatially due to the transient nature of the tests. A parameter describing the nonisothermal cavity wall temperature variation was defined in order to compare with isothermal cavity heat transfer results. Results. Results showed that the cavity Nusselt number, based on a cavity averaged temperature, was insensitive to the transient development of nonisothermal conditions within the cavity. Comparison of cavity-average Nusselt number for the current study, where the Rayleigh number ranged from 5×106 to 2×108, to data from the literature showed good agreement. Cavity-average Nusselt number relations for inclination angles of 0, 45, and 90deg in the form of NuH,cav=CRa1∕3 resulted in coefficients of 0.091, 0.105, 0.093, respectively. The 45deg inclination angle orientation yielded the largest Nusselt numbers, which was similar to previous literature results. Trends in the local plate Nusselt numbers were examined and found similar to data from the literature.

Heat Transfer From an Isothermally Heated Flat Surface Due to Confined Laminar Twin Oblique Slot-Jet Impingement

2015 ◽  
Vol 7 (3) ◽  
Author(s):  
Muhammad A. R. Sharif
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Average Nusselt Number ◽  
Flat Surface ◽  
Heated Surface ◽  
Jet Impingement ◽  
Impingement Angle ◽  
Exit Velocity ◽  
Flow Domain ◽  
Impingement Surface

Convective heat transfer from a heated flat surface due to twin oblique laminar slot-jet impingement is investigated numerically. The flow domain is confined by an adiabatic surface parallel to the heated impingement surface. The twin slot jets are located on the confining surface. The flow and geometric parameters are the jet exit Reynolds number, distance between the two jets, distance between the jet exit and the impingement surface, and the inclination angle of the jet to the impingement surface. Numerical computations are done for various combinations of these parameters, and the results are presented in terms of the streamlines and isotherms in the flow domain, the distribution of the local Nusselt number along the heated surface, and the average Nusselt number at the heated surface. It is found that the peak and the average Nusselt number on the hot surface mildly decreases and the location of the stagnation point and the peak Nusselt number gradually moves downstream as the impingement angle is decreased from 90 deg. The heat transfer distribution from the impingement surface gets more uniform as the impingement angle is reduced to 45 deg and 30 deg at lager jet-to-plate distance (4–8) with a corresponding overall heat transfer reduction of about 40% compared to the normal impinging jet case. The specified jet exit velocity profile boundary condition has considerable effect on the predicted Nusselt number around the impingement location. Fully developed jet exit velocity profile correctly predicts the Nusselt number when compared to the experimental data.

Numerical Simulation of Steady State Heat Transfer in a Microchannel With Obstruction

2007 ◽  
Author(s):  
Phaninder Injeti ◽  
Muhammad M. Rahman
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Local Nusselt Number ◽  
Average Nusselt Number ◽  
Isothermal Condition ◽  
Fabrication Process ◽  
Two Dimensional ◽  
Micro Electro Mechanical Systems ◽  
R 134A ◽  
Steady State Heat Transfer

Effects of obstructions on the heat transfer in a microtube and a two-dimensional microchannel were investigated. The obstructions normally arise during the fabrication process on the inner surface of the microtube or microchannel. Various shapes and sizes of the obstruction were considered. The shapes that were modeled were rectangular, triangular, and semicircular obstructions. Calculations were done for incompressible flow of a Newtonian fluid with developing momentum and thermal boundary layers. Equations governing the conservation of mass, momentum and energy were solved for an isothermal condition at the wall. Comparison with no obstruction case was made. It was found that for microtubes, the local Nusselt number increases significantly at the obstruction whereas the average Nusselt number for the whole tube comes out somewhat lower. For two-dimensional microchannels, both local Nusselt number at the obstruction as well as average Nusselt number for the entire tube came out to be higher. The working fluids that were investigated were water and refrigerant R-134a. The results presented in the paper demonstrates that in microchannels and microtubes that are widely used in micro electro mechanical systems (MEMS), small defects in the fabrication process may lead to large changes in local as well as global performance of the device.

Transient Thermal Management Using a Confined Jet Impingement of Liquid Ammonia

2003 ◽  
Author(s):  
Ryan M. Mead ◽  
Muhammad M. Rahman
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Transfer Coefficient ◽  
Steady State ◽  
Average Nusselt Number ◽  
Liquid Ammonia ◽  
Plate Thickness ◽  
Average Heat Transfer Coefficient ◽  
Steady State Condition ◽  
Average Heat

This paper introduces the results of transient heat transfer involving a jet of liquid ammonia perpendicularly on a solid substrate of finite thickness containing discrete electronic sources on the opposite surface. The jet was confined by using a cover plate to prevent any evaporation or loss of ammonia during the heat transfer process. The numerical simulation considered both the solid and fluid regions as a conjugate problem. The equations solved in the liquid region included the conservation of mass, conservation of energy, and conservation of momentum. For the solid region, only the heat conduction equation was solved. Computed results included the temperature distribution, local and average heat transfer coefficient, and local and average Nusselt number at the solid-fluid interface. Some of the parameters such as the jet velocity, plate thickness, and plate material were altered to examine the effect that they had on the problem. It was found that the average heat transfer coefficient and a average Nusselt number were high at the initial stages of the transient process and decreased steadily with time until it reached the steady condition. As the plate thickness decreased, and as the jet velocity increased, it was observed that the time it took to reach the steady state condition declined. The time it took to reach steady state condition did not change significantly for different plate materials. However, it did change noticeably for different plate thickness and different Reynolds number.

An Experimental Investigation of Forced Convection Heat Transfer From an Isothermal V-Shaped Plate

2010 ◽  
Author(s):  
Tooraj Yousefi ◽  
Saeed Ebrahimi ◽  
Masood Bigharaz ◽  
Sajjad Mahmoodi Nezhad
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Local Nusselt Number ◽  
Average Nusselt Number ◽  
Convection Heat Transfer ◽  
Jet Impingement ◽  
Reynolds Numbers ◽  
Low Reynolds Numbers ◽  
Plate Spacing ◽  
Internal Surfaces

An experimental study has been carried out to investigate heat transfer characteristics on internal surfaces of a V-shaped plate exposed to a slot jet impingement of air. A square-edged nozzle is mounted parallel with V-shaped plate axis and jet flow impinges on the bottom of the V-shaped plate. The study is focused on Rayleigh number 159000, angle of V-shaped plate ranging from 22.5 to 45 degree, low Reynolds numbers ranging from 29.05 to 60.41, and slot-to-(V-shaped plate) spacing from 17 to 21 of the slot width. A Mach-Zehnder interferometer is used for measurement of local Nusselt number on the V-shaped plate. It is observed that the local Nusselt number and average Nusselt number decrease with increasing the jet spacing and increase with increasing the Reynolds number. Also the local Nusselt number and average nusselt number increase with rising the angle of V-shaped plate.

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