Showing papers on "Heat transfer coefficient published in 1979"

Theoretical prediction of effective heat transfer parameters in packed beds

Abstract: A theory for predicting the effective axial and radial thermal conductivities and the apparent wall heat transfer coefficient for fluid flow through packed beds is derived from a two-phase continuum model containing the essential underlying and independently measurable heat transfer processes. The theory is shown to explain much of the confused literature and pinpoints the remaining major areas of uncertainty, further investigation of which is needed before secure prediction is possible.

Instabilities of convection rolls in a fluid of moderate Prandtl number

Abstract: The instabilities of two-dimensional convection rolls in a horizontal fluid layer heated from below are investigated in the case when the Prandtl number is seven or lower. Two new mechanisms of instability are described theoretically as well as experimentally. The knot instability causes the transition to spoke-pattern convection at higher Rayleigh numbers while the skewed varicose instability accomplishes a change to larger horizontal wavelengths of the convection rolls. Both instabilities disappear in the limits of small and large Prandtl number. Although the experimental methods fail in realizing closely the infinitely conducting boundaries assumed in the theory, the observations agree in all qualitative aspects with the theoretical predictions.

An Excess Enthalpy Flame Theory

Abstract: A simple idea is proposed to produce an excess enthalpy flame by inserting a porous solid of high thermal conductivity into the one-dimensional flame zone. Heat is recirculated internally through the solid from the downstream high temperature region to the upstream low temperature region and large excess enthalpy is produced at the head of the reaction zone. The potentiality of the proposed artificially modified flame is analyzed on the basis of the simplified one-dimensional flame theory. It is found that in this flame the mass flow rate is not the eigenvalue but becomes a mere parameter. The heat transfer coefficient between the solid and the reacting gas is another parameter. With these additional two parameters the controllability over the flame characteristics increases remarkably. The analysis reveals several attractive characteristics of the flame and that the proposed idea is promising to burn mixtures of low heat content in a simple combustion system.

Heat Transfer Characteristics for Inline and Staggered Arrays of Circular Jets with Crossflow of Spent Air

Abstract: Heat transfer characteristics were measured for two dimensional arrays of jets impinging on a surface parallel to the jet orifice plate. The impinging flow was constrained to exit in a single direction along the channel formed by the jet plate and the heat transfer surface. Both mean Nusselt numbers and streamwise Nusselt number profiles are presented as a function of Reynolds number and geometric parameters. These are the streamwise and transverse hole spacings ranging from 5 to 10 and 4 to 8 jet orifice diameters, respectively; the channel height ranging from 1 to 6 diameters; and the hole pattern which includes both inline and staggered arrays. The results show that significant periodic variations occur in the streamwise Nusselt number profiles, persisting downstream for at least ten rows of jet holes. Channel height can have a significant effect on the chordwise profiles, smoothed across the periodic variations. For the smaller channel heights, Nusselt numbers first decrease and then increase downstream. Where significant differences exist, inline hole patterns provide better heat transfer than staggered ones. These and other effects of the geometric parameters are presented and discussed.

The turbulent heat flux from arctic leads

Abstract: The turbulent transfer of heat from Arctic leads in winter is one of the largest terms in the Arctic heat budget. Results from the AIDJEX Lead Experiment (ALEX) suggest that the sensible component of this turbulent heat flux can be predicted from bulk quantities. Both the exponential relation N = 0.14R x 0.72 and the linear relation N = 1.6 × 10−3 R x+ 1400 fit our data well. In these, N is the Nusselt number formed with the integrated surface heat flux, and R x is the Reynolds number based on fetch across the lead. Because of the similarity between heat and moisture transfer, these equations also predict the latent heat flux. Over leads in winter, the sensible heat flux is two to four times larger than the latent heat flux. The internal boundary layer (IBL) that develops when cold air encounters the relatively warm lead is most evident in the modified downwind temperature profiles. The height of this boundary layer, δ, depends on the fetch, x, on the surface roughness of the lead, z 0 and on both downwind and upwind stability. A tentative, empirical model for boundary layer growth is % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaadaWcaaqaaiabes% 7aKbqaaiaadQhadaWgaaWcbaGaaGimaaqabaaaaOGaeyypa0JaeqOS% di2aaeWaaeaacqGHsisldaWcaaqaaiaadQhadaWgaaWcbaGaaGimaa% qabaaakeaacaWGmbaaaaGaayjkaiaawMcaamaaCaaaleqabaGaaGim% aiaac6cacaaI4aaaaOWaaeWaaeaadaWcaaqaaiaadIhaaeaacaWG6b% WaaSbaaSqaaiaaicdaaeqaaaaaaOGaayjkaiaawMcaamaaCaaaleqa% baGaaGimaiaac6cacaaI0aaaaaaa!472D!\[\frac{\delta }{{z_0 }} = \beta \left( { - \frac{{z_0 }}{L}} \right)^{0.8} \left( {\frac{x}{{z_0 }}} \right)^{0.4} \] where L is the Obukhov length based on the values of the momentum and sensible heat fluxes at the surface of the lead, and Β is a constant reflecting upwind stability. Velocity profiles over leads are also affected by the surface nonhomogeneity. Besides being warmer than the upwind ice, the surface of the lead is usually somewhat rougher. The velocity profiles therefore tend to decelerate near the surface, accelerate in the mid-region of the IBL because of the intense mixing driven by the upward heat flux, and rejoin the upwind profiles above the boundary layer. The profiles thus have distinctly different shapes for stable and unstable upwind conditions.

Heat and mass transfer to and from surfaces with dense vegetation or similar permeable roughness

Abstract: A differential equation is obtained to describe the concentration of passive admixtures (water vapor, sensible heat, pollutants, CO2, etc.) of turbulent flow inside a dense and uniform vegetational canopy. The profiles of eddy diffusivity, wind speed and shear stress are assumed to be exponential decay functions of depth below the top of the canopy. This equation is solved for the case of a vegetation with constant concentration of the admixture at the foliage surfaces. The solution is used to formulate bulk mass or heat transfer coefficients, which can be applied to practical problems involving surfaces covered with a vegetation or with similar porous or fibrous roughness elements. The results are shown to be consistent with experimental data presented by Chamberlain (1966), Garratt and Hicks (1973) and Garratt (1978). Calculations with the model illustrate that, as compared to its behavior over surfaces with bluff roughness elements, ln(z 0/z 0c ) (wherez 0 is the momentum roughness andz 0c , the scalar roughness) for permeable roughness elements is relatively insensitive tou * and practically independent ofz 0.

Combined heat and mass transfer in natural convection on inclined surfaces

Abstract: The combined heat and mass transfer characteristics of natural convection flow along inclined surfaces are studied analytically. The buoyancy forces arise from both temperature and concentration variations in the fluid. In the analysis, the diffusion-thermo and thermo-diffusion effects are neglected, as are the interfacial velocities resulting from mass diffusion. The surfaces are either maintained at a uniform temperature/concentration or subjected to a uniform heat/mass flux. The important parameters of the problem include Prandtl and Schmidt numbers, thermal and concentration Grashof numbers, the relative buoyancy force effect between species and thermal diffusion, and the angle of inclination from the vertical. Numerical results are presented for diffusion of common species into air and water. For both heating/diffusing conditions, the wall shear stress and the local Nusselt number are found to increase and decrease as the buoyancy force from species diffusion assists and opposes, respectively, the th...

Heat Transfer Between a Gas Fluidized Bed and Immersed Tubes

Abstract: Publisher Summary Bed–to–surface heat transfer can be calculated with good reliability, if particle-residence times and their population near the surface are known The mechanism of heat transfer between boiler tubes and the fluidized bed is very complicated because of the many fluidized-bed variables and the variations in heat-transfer tube design Correlations are empirically developed on the basis of the observed data with limited attention to the mechanisms of the heat-transfer process This chapter reviews the design application of local as well as total heat-transfer coefficients for horizontal and slanted tubes, and for vertical tubes Single tubes and tube banks in different orientations to the bed are also considered The fundamental mechanisms of wall–to–bed heat transfer are reviewed and the criteria for evaluation of correlations for full-scale application are examined Particle movement is a complex function of the particle and gas properties, fluidization velocity, gas distributor, and geometry of the bed This approach generally gives the designer a good degree of assurance, requires a judicious choice of a representative system as well as the expense of pilot scale development The consideration must be given to the unsteady-state conduction component, the radiation component, and the gas convective component in boiler design

Some simplified expressions for the thermal conductivity in an external field

Abstract: For a polyatomic gas simple expressions have been derived for the thermal conductivity as well as for the variation of the thermal conductivity as a function of an external field. This has been achieved by using the total heat transport as a trial function in the kinetic theory instead of the usual approach in which translational and rotational heat transport are treated separately. For a number of gases a numerical comparison is made between both methods.

Review paper: Heat transfer between liquid helium and solids below 100 mK

Abstract: The many experimental determinations of heat transfer between liquid helium and solids are reviewed and compared with the existing theories. Generally heat transfer is a complex process at low temperatures, involving parallel heat paths with several resistances in series in each path. The standard theories for the individual resistances are reviewed and as far as possible a definite value of the expected resistance obtained. The experiments have been considered in five groups: cerium magnesium nitrate in liquid3He, cerium magnesium nitrate in dilute3He in liquid4He, metals in liquid3He, metals in dilute3He in liquid4He, and miscellaneous experiments. Where appropriate, the experimental results have been reevaluated in terms of standard models for heat transfer and presented as resistance versus temperature on diagrams showing the theoretical predictions. Between 20 and 100 mK many measurements show good agreement with acoustic mismatch theory for thermal boundary resistance. Below 20 mK most experiments have been made with finely divided solids (sintered metals or powdered paramagnetic salts); invariably the resistance has been anomalously low, with the metal/3He systems showingR ∫ T −1 and the metal/dilute3He in4He systems showingR ∫ T −3. The cerium magnesium nitrate/helium experiments have shown a temperature—independent resistance and sometimes anR ∫ T ~1 dependence. These resistances have been attributed to the spin-lattice resistance in cerium magnesium nitrate. Evidence for heat transfer by magnetic coupling has been reviewed and it is concluded that the positive evidence has other explanations, while the lack of dependence upon helium pressure,3He phase, and large magnetic fields is strong negative evidence. If the disagreement of experimental results with standard theory is not to be attributed to magnetic coupling, then several theoretical questions remain to be answered; these questions are posed. Basically, at low temperatures the excitations in solids and liquid helium have wavelengths and mean free paths much larger than the size of the finely divided particles or pores between the particles. Thus theories for bulk solids and bulk liquid helium are not appropriate for describing excitations and their interactions at these low temperatures.

Dual-pull air cooling for a computer frame

Abstract: An air cooling arrangement for a column of integrated circuit modules is provided in which a pair of air moving devices are included, one located at the top and one at the bottom of the column each pulling air through the column. A heat sink having opposite side air outlets is attached to each of the modules and extends from the top side thereof. A cover plate having openings therein indexed with each of the heat sinks allows air to pass through the openings into the heat sinks. An air flow guide is located at each air outlet side of each heat sink and extends from the cover plate vertically into each heat sink to approximately the half way point to provide a vertical impinging air flow pattern which yields a high heat transfer coefficient. Air distribution ducts are arranged on opposite sides of the heat sinks parallel to the column and connected to the opposite side of air outlets of the heat sinks via air passages thereby providing a dividing of the flow across each heat sink.

Heat capacity measurements by means of thermal relaxation method in medium temperature range

Abstract: The thermal relaxation method is used to measure heat capacities of small samples in the temperature range around room temperature. The calorimeter is designed to measure under heat exchange gas and a steplike excess heat is applied to the sample by irradiation from a lamp.

On the computation of heat transfer coefficients from energy-balance gradients on a glacier

Abstract: The turbulent flux of sensible heat in the energy balance of a glacier surface is assumed to be proportional to the temperature difference between the glacier surface and the atmosphere at the same level but outside the thermal influence of the glacier. The factor of proportionality between them is first explained in terms of friction velocity, roughness height, and stability function of the logarithmic wind and temperature profile. It is then derived from climatological records and measurements of the energy balance and its altitudinal gradients at Hintereisferner. Examples of the energy-balance components and their change with altitude are given for the entire ablation season as well as for short periods. The heat transfer coefficients derived have a mean value of 1.7±0.2 MJ m−2 d−1 K−1 (40 ± 5 ly d−1 K−1).

Forced Convection Heat Transfer on Heated Bottom Surface of a Cavity

Abstract: Experiments to measure the heat transfer characteristics for various cavities situated at a duct-wall were performed. Flow visualization, measurements of pressure and temperature distributions on the heated bottom surface of cavity were carried out. It was observed that the effects of main flow stream, reattachment of separated flow, and vortex flow in the cavity on heat transfer unexpectedly large. It was found that heat transfer did not always decrease monotonously with an increase of aspect (depth-width) ratio D/W, in the flow range of laminar to turbulent. Correlations between Num and Rew were made in laminar and turbulent heat transfer ranges.

Prediction of the Bulk Temperature in Spur Gears Based on Finite Element Temperature Analysis

Abstract: The temperature distribution in spur gears operating in a state of thermal equilibrium is solved by using a finite element method. The effects of various dimensionless parameters on bulk temperature are shown. A table is provided which can be used to predict the bulk temperature on gear teeth, once the heat transfer coefficients and frictional heat input is estimated. Theoretical results for estimating heat transfer coefficients and frictional heat are also summarized. Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Kansas City, Missouri, October 3–5, 1977

Natural convection heat transfer characteristics of flat plate enclosures

Abstract: Heat transfer by natural convection in rectangular enclosures has been experimentally studied using interferometric techniques. The effects of Grashof number, tilt angle, and aspect ratio on both the local and average heat transfer coefficients have been determined. The Grashof number range tested was 4 x 10/sup 3/ to 3.1 x 10/sup 5/, and the aspect ratio (ratio of enclosure length to plate spacing) varied between 9 and 36. The angles of tilt of the enclosure with respect to the horizontal were 45, 60, 75 and 90 deg. Correlations are developed for both local and average Nusselt number over the range of test variables. The effect of tilt angle is found to reduce the average heat transfer by about 18 percent from the value of 45 deg to that at 90 deg. No significant effect of aspect ratio over the range tested was found. A method for characterizing the flow regimes that is based on heat transfer mechanisms is proposed.