Effects of Averaging the Heat Transfer Coefficient on Predicted Material Temperature and Its Gradient

2016 ◽  
Vol 139 (2) ◽  
Author(s):  
Chien-Shing Lee ◽  
Tom I-P. Shih ◽  
Kenneth Mark Bryden
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Temperature Gradient ◽  
Transfer Coefficient ◽  
Flat Plate ◽  
Biot Number ◽  
Maximum Temperature ◽  
Gradual Transition ◽  
The Heat Transfer Coefficient ◽  
Maximum Temperature Gradient

The heat transfer coefficient (HTC) is often averaged spatially when designing heat exchangers. Since the HTC could vary appreciably about a heat transfer enhancement feature such as a pin fin or a rib, it is of interest to understand the effects of averaging the HTC on design. This computational study examines those effects via a unit problem—a flat plate of thickness H and length L, where L represents the distance between pin-fins or ribs. This flat plate is heated on one side, and cooled on the other. Variable HTC is imposed on the cooled side—a higher HTC (hH) over LH and a lower HTC (hL) over LL = L − LH. For this unit problem, the following parameters were studied: abrupt versus gradual transition between hH and hL, hH/hL, LH/L, and H/L. Results obtained show that if the averaged HTC is used, then the maximum temperature in the plate and the maximum temperature gradient in the plate can be severely underpredicted. The maximum temperature and the maximum temperature gradient can be underpredicted by as much as 36.3% and 542%, respectively, if the Biot number is less than 0.1 and as much as 13.0% and 570% if the Biot number is between 0.25 and 0.4. A reduced-order model was developed to estimate the underpredicted maximum temperature.

Nanoparticle aggregation kinematics on the quadratic convective magnetohydrodynamic flow of nanomaterial past an inclined flat plate with sensitivity analysis

2021 ◽  
pp. 095440892110562
Author(s):  
AS Sabu ◽  
Joby Mackolil ◽  
B Mahanthesh ◽  
Alphonsa Mathew
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Sensitivity Analysis ◽  
Heat Transfer Coefficient ◽  
Heat Source ◽  
Transfer Coefficient ◽  
Flat Plate ◽  
Inclined Plate ◽  
The Impact ◽  
The Heat Transfer Coefficient

The study focuses on the aggregation kinematics in the quadratic convective magneto-hydrodynamics of ethylene glycol-titania ([Formula: see text]) nanofluid flowing through an inclined flat plate. The modified Krieger-Dougherty and Maxwell-Bruggeman models are used for the effective viscosity and thermal conductivity to account for the aggregation aspect. The effects of an exponential space-dependent heat source and thermal radiation are incorporated. The impact of pertinent parameters on the heat transfer coefficient is explored by using the Response Surface Methodology and Sensitivity Analysis. The effects of several parameters on the skin friction and heat transfer coefficient at the plate are displayed via surface graphs. The velocity and thermal profiles are compared for two physical scenarios: flow over a vertical plate and flow over an inclined plate. The nonlinear problem is solved using the Runge–Kutta-based shooting technique. It was found that the velocity profile significantly decreased as the inclination of the plate increased on the other hand the temperature profile improved. The heat transfer coefficient decreased due to the increase in the Hartmann number. The exponential heat source has a decreasing effect on the heat flux and the angle of inclination is more sensitive to the heat transfer coefficient than other variables. Further, when radiation is incremented, the sensitivity of the heat flux toward the inclination angle augments at the rate 0.5094% and the sensitivity toward the exponential heat source augments at the rate 0.0925%. In addition, 41.1388% decrement in wall shear stress is observed when the plate inclination is incremented from [Formula: see text] to [Formula: see text].

Evaluation of Heating Process of Apple, Eggplant, Zucchini and Potato by Means of Their Thermal Properties

2016 ◽  
Author(s):  
Hilario Terres ◽  
Sandra Chavez ◽  
Raymundo Lopez ◽  
Arturo Lizardi ◽  
Araceli Lara
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Thermal Properties ◽  
Transfer Coefficient ◽  
Biot Number ◽  
Heating Process ◽  
Energy Devices ◽  
Heating Device ◽  
The Heat Transfer Coefficient ◽  
Transitory State

In this work, the heating process for apple, eggplant, zucchini and potato by means of evaluation of their thermal properties and the Biot number determined in experimental form is presented. The heating process is carried out using a solar cooker box-type as heating device. The thermal experimental properties determined are conductivity (k), density (D), specific heat (C), diffusivity (Dif) and the Biot number (Bi) for each product evaluated. In the experimentation, temperatures for center and surface in each product and water were measured in controlled conditions. For those measures, a device Compact Fieldpoint and thermocouples placed in the points studied were used. By using correlations with temperature as function, k, D and C were calculated, while by using equations in transitory state for the products modeled as sphere and cylinder was possible to estimate the Biot number after calculation of the heat transfer coefficient for each case. Results indicate the higher value for k, C and Dif correspond to zucchini (0.65 W/m °C, 4084.5 J/kg °C, 1.5 × 10−7 m2), while higher value for D correspond to potato (1197.5 kg/m3). The lowest values for k and C were obtained for potato (0.59 W/m °C, 3658.3 J/kg °C) while lowest values for D and Dif, correspond to zucchini (998.2 kg/m3) and potato (1.45 × 10−7 m2/s) respectively. The maximum and minimum values for Bi corresponded to potato (21.4) and zucchini (0.41) in respective way. The results obtained are very useful in applications for solar energy devices, where estimates for properties are very important to generate new results, for example, numerical simulations. Also, results could be used to evaluate the cooking power in solar cookers when the study object is oriented in that direction.

Heat Transfer in the Separated and Reattached Flow on a Blunt Flat Plate

1974 ◽  
Vol 96 (4) ◽  
pp. 459-462 ◽  
Author(s):  
Terukazu Ota ◽  
Nobuhiko Kon
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flat Plate ◽  
Leading Edge ◽  
Two Dimensional ◽  
Separated And Reattached Flow ◽  
The Heat Transfer Coefficient ◽  
Blunt Leading Edge ◽  
Made In

Heat transfer measurements are made in the separated, reattached, and redeveloped regions of the two-dimensional air flow on a flat plate with blunt leading edge. The flow reattachment occurs at about four plate thicknesses downstream from the leading edge and the heat transfer coefficient becomes maximum at that point and this is independent of the Reynolds number which ranged from 2720 to 17900 in this investigation. The heat transfer coefficient is found to increase sharply near the leading edge. The development of flow is shown through the measurements of the velocity and temperature in the separated, reattached, and redeveloped regions.

Wind-Related Heat Transfer Coefficient for Flat-Plate Solar Collectors

1987 ◽  
Vol 109 (2) ◽  
pp. 108-110 ◽  
Author(s):  
S. Shakerin
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flat Plate ◽  
Separation Bubble ◽  
Convective Heat Transfer Coefficient ◽  
Leading Edge ◽  
The Heat Transfer Coefficient ◽  
Good Agreement

Experiments were performed to evaluate the convective heat transfer coefficient for a flat plate mounted in a wooden model of a roof of a building. The experiments were carried out in a closed-circuit wind tunnel and included parametric adjustments of the roof tilt and Reynolds number, based on the length of the plate. The roof tilt was set at 0, 30, 45, 60, and 90 degrees and the Reynolds number ranged from 58,000 to 250,000. A transient, one lump, thermal approach was used for heat transfer calculations. Due to a separation bubble at the leading edge of the model, i.e., the roof, at angles of attack of less than 40 degrees, the flow became turbulent after reattachment. This resulted in a higher heat transfer than previously reported in the literature. At higher angles of attack, the flow was not separated at the leading edge and remained laminar. The heat transfer coefficient for higher angles of attack, i.e., α > 40 deg, was found to be approximately independent of the angle of attack and in good agreement with the previously published results.

An Inverse Method for Drying at High Mass Transfer Biot Number

2003 ◽  
Author(s):  
Gligor H. Kanevce ◽  
Ljubica P. Kanevce ◽  
George S. Dulikravich ◽  
Marcelo J. Colac¸o
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Mass Transfer Coefficient ◽  
Heat And Mass Transfer ◽  
Biot Number ◽  
Temperature Sensitivity ◽  
Sensitivity Coefficient ◽  
The Heat Transfer Coefficient

The inverse problem of using temperature measurements to estimate the moisture content and temperature-dependent moisture diffusivity together with the heat and mass transfer coefficients is analyzed in this paper. In the convective drying practice, usually the mass transfer Biot number is very high and the heat transfer Biot number is very small. This leads to a very small temperature sensitivity coefficient with respect to the mass transfer coefficient when compared to the temperature sensitivity coefficient with respect to the heat transfer coefficient. Under these conditions the relative error of the estimated mass transfer coefficient is high. To overcome this problem, in this paper the mass transfer coefficient is related to the heat transfer coefficient through the analogy between the heat and mass transfer processes in the boundary layer. The resulting parameter estimation problem is then solved by using a hybrid constrained optimization algorithm OPTRAN.

scholarly journals The Effect of Density Ratio on the Heat Transfer Coefficient From a Film Cooled Flat Plate

1989 ◽  
Author(s):  
H. D. Ammari ◽  
N. Hay ◽  
D. Lampard
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flat Plate ◽  
Strong Dependence ◽  
Density Ratio ◽  
Polymer Surface ◽  
Operating Conditions ◽  
Density Ratios ◽  
The Heat Transfer Coefficient

The effect of density ratio of cooling films on the heat transfer coefficient on a flat plate is investigated using a heat-mass transfer analogy. The experimental technique employed uses a swollen polymer surface and laser holographic interferometry. A density ratio of 1.0 was achieved using air as the injectant. Density ratios of 1.38 and 1.52, representative of turbine operating conditions, were obtained by using foreign gases. The coolant fluids were injected at various blowing rates through a single normal hole or through a row of holes spaced at three-diameter intervals, and inclined at 35° or 90° to the mainstream direction. The experiments were conducted under isothermal conditions in a subsonic, zero mainstream pressure gradient turbulent boundary layer. The results indicated large differences in behaviour between the two injection angles. For normal injection, the heat transfer coefficient at a fixed blowing parameter was insensitive to the variation of density ratio, whereas for 35° injection strong dependence was observed. Scaling parameters for the heat transfer data have been proposed so that use can be made of data obtained at density ratios not representative of gas turbine practice. In addition, a correlation for normal injection data has been formulated.

Effects of Averaging the Heat-Transfer Coefficient on the Predicted Material Temperature Distribution

2015 ◽  
Author(s):  
T. I-P. Shih ◽  
C.-S. Lee ◽  
K. M. Bryden
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Temperature Distribution ◽  
Transfer Coefficient ◽  
Computational Study ◽  
Maximum Temperature ◽  
Bulk Temperature ◽  
Spanwise Direction ◽  
Gas Temperature ◽  
The Heat Transfer Coefficient

The heat-transfer coefficient (HTC) in internal-coolant passages can vary appreciably about a heat-transfer enhancement feature such as a pin fin, a rib, and a concavity because of stagnation regions and wakes about the enhancement feature. However, the computed or measured HTC is often averaged spatially in the spanwise direction or over some region when used in the design of cooling strategies. Since the variation in the HTC could be a factor of eight or more about an enhancement feature, it is of interest to understand the effects of averaging the HTC on the predicted temperature distribution in the solid subjected to the heating and cooling. In this computational study, a flat plate of thickness H (1 mm) and length L = 20H is heated on one side by either a constant heat flux (68 W/cm2) or a constant HTC (1,167.2 W/m2-K) and a constant hot-gas temperature (1,482 °C). On the cooled side, the free stream or bulk temperature is kept constant (400 °C) and the average HTC (1,442.5 W/m2-K) is kept constant as well. This average HTC on the cooled side is the average of a higher HTC (hH) and a lower HTC (hL). Two types of changes from hH to hL are considered — abrupt (or step) and gradual. When the HTC changes abruptly, hH is imposed over LH, and hL is imposed over LL=L–LH. When the HTC changes gradually from hH to hL, hH is imposed from from x = 0 to LH/2, and hL is imposed from x = 3LH/2 to L with a smooth variation in the HTC to connect hH and hL. Results obtained show that when the averaged HTC is used, the maximum temperature in the plate is 900 °C on the heated side of the plate. However, if the variation in the HTC is accounted for, then the maximum temperature in the plate could be as high as 1.363 times the maximum temperature predicted by assuming an averaged HTC. Also, for the range of parameters studied, the difference in the maximum and minimum temperature in the plate can increase by a factor of 16, which strongly affects thermal stress.

Scaling of Thermal Positioning in Microscale and Nanoscale Bridge Structures

2012 ◽  
Vol 134 (10) ◽  
Author(s):  
Elham Maghsoudi ◽  
Michael James Martin
Keyword(s):  
Heat Transfer ◽  
Silicon Carbide ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Biot Number ◽  
Cvd Diamond ◽  
Bridge Structures ◽  
The Heat Transfer Coefficient ◽  
Dimensionless Displacement ◽  
The Continuum

Heat transfer in a thermally positioned doubly clamped bridge is simulated to obtain a universal scaling for the behavior of microscale and nanoscale bridge structures over a range of dimensions, materials, ambient heat transfer conditions, and heat loads. The simulations use both free molecular and continuum models to define the heat transfer coefficient, h. Two systems are compared: one doubly clamped beam with a length of 100 μm, a width of 10 μm, and a thickness of 3 μm, and a second beam with a length of 10 μm, a width of 1 μm, and a thickness of 300 nm, in the air at a pressure from 0.01 Pa to 2 MPa. The simulations are performed for three materials: crystalline silicon, silicon carbide, and chemical vapor deposition (CVD) diamond. The numerical results show that the displacement and the response of thermally positioned nanoscale devices are strongly influenced by ambient cooling. The displacement depends on the material properties, the geometry of the beam, and the heat transfer coefficient. These results can be collapsed into a single dimensionless center displacement, δ* = δk/q″αl2, which depends on the Biot number and the system geometry. The center displacement of the system increases significantly as the bridge length increases, while these variations are negligible when the bridge width and thickness change. In the free molecular model, the center displacement varies significantly with the pressure at high Biot numbers, while it does not depend on cooling gas pressure in the continuum case. The significant variation of center displacement starts at Biot number of 0.1, which occurs at gas pressure of 27 kPa in nanoscale. As the Biot number increases, the dimensionless displacement decreases. The continuum-level effects are scaled with the statistical mechanics effects. Comparison of the dimensionless displacement with the thermal vibration in the system shows that CVD diamond systems may have displacements that are at the level of the thermal noise, while silicon carbide systems will have a higher displacement ratios.

Effects of Cooling Films on the Heat Transfer Coefficient on a Flat Plate With Zero Mainstream Pressure Gradient

1985 ◽  
Vol 107 (1) ◽  
pp. 105-110 ◽  
Author(s):  
N. Hay ◽  
D. Lampard ◽  
C. L. Saluja
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flat Plate ◽  
Polymer Surface ◽  
Flux Ratio ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Transfer Coefficients ◽  
The Heat Transfer Coefficient

The influence of injection of cooling films through a row of holes on the heat transfer coefficient on a flat plate is investigated for a range of mass flux ratio using a heat-mass transfer analogy. Injection angles of 35 deg and 90 deg are covered. The experimental technique employed uses a swollen polymer surface and laser holographic interferometry. The results presented show the change in local heat transfer coefficient over the no-injection values at the centerline and off-centerline locations for various streamwise stations. The effect of injection on laterally averaged heat transfer coefficients is also assessed.

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