Showing papers on "Heat transfer coefficient published in 1985"

Inverse Heat Conduction: Ill-Posed Problems

Abstract: Description of the Inverse Heat Conduction Problem Exact Solutions of the Inverse Heat Conduction Problem Approximate Methods for Direct Heat Conduction Problems Inverse Heat Conduction Estimation Procedures Inverse Convolution Procedures Difference Methods for Solution of the One Dimensional Inverse Heat Conduction Problem Multiple Heat Flux Estimation Heat Transfer Coefficient Estimation Index

Heat transfer enhancement in channels with turbulence promoters

Abstract: Repeated rib-roughness elements have been used in advanced turbine cooling designs to enhance the internal heat transfer Often the ribs are perpendicular to the main flow direction so that they have an angle-of-attack of 90 degrees The objective of this investigation was to determine the effect of rib angle-of-attack on the pressure drop and the average heat-transfer coefficients in the fully developed turbulent air flow in a square duct with two opposite rib-roughened walls for Reynolds numbers varied from 7,000 to 90,000 The rib height-to-equivalent diameter ratio was kept at a constant value of 0063, the rib pitch-to-height ratio was varied from 10 to 20, and the rib angle-of-attack (alpha) was varied from 90 to 60 deg to 45 to 30 deg, respectively The thermal-performance comparison indicated that the increased heat conductance for the rib with an oblique angle to the flow (alpha = 45 deg - 30 deg) was about 10-20 percent higher than the rib with a 90 deg angle to the flow, and the pumping power requirement for the angled rib was about 20-50 percent lower than the transverse rib Semi-empirical correlations for friction factor and heat-transfer coefficients were developed to account for rib spacing and rib angle The correlations can be used in the design of turbine-blade cooling passages

Thermal evolution models for the Earth

Abstract: Thermal evolution models for the earth which are based on a parameterization of the convective heat transport are critically reexamined. Traditionally, it has been assumed that the temperature dependence of the mantle viscosity implies that internal temperature and convective heat loss are strongly coupled. Recent numerical work on the heat transport by variable viscosity convection demonstrates that the dependence of the heat flow on the mantle temperature may in fact be much weaker than expected. I compare thermal evolution models with strong and weak dependence of the heat loss on the temperature. With the weaker dependence, plate velocities and heat flow in the Archean were not more than 50% higher than today, while with the strong dependence, much larger differences are predicted. In the former case the Archaean mantle temperatures are somewhat higher, and the present-day ratio of radioactive heat production over heat loss (Urey ratio) is 50% or slightly less. The Urey ratios in the traditional parameterized evolution models are >70%. The predictions of both kinds of models are compared with the independent geological, geochemical, and palaeomagnetic evidence. Although this evidence is subject to some uncertainties, it favors in every case the evolution models based on a weak coupling of heat loss to the interior temperature.

Use of laboratory data in freeze drying process design: heat and mass transfer coefficients and the computer simulation of freeze drying.

Abstract: Freeze drying process development normally proceeds via an empirical “trial and error” experimental approach which is both time consuming and uncertain in reliable extrapolation to production equipment. This research describes the use of phenomenological theory, with key parameters determined by laboratory experiments, to guide the experimental program to optimize the primary drying stage of the process for a given product/container combination. The theoretical description of primary drying is a problem in coupled heat and mass transfer which can be satisfactorily described using a steady-state model where the heat flow is given by the product of the mass flow and the heat of sublimation. Generally, mass transfer is impeded by three barriers or resistances: resistance of the dried product layer, resistance of the semi-stoppered vial, and resistance of the chamber. Resistance, defined as a ratio of pressure difference to mass flow, is experimentally determined for each barrier. The resistance of the dried product normally increases with time as the thickness of dried product increases and typically accounts for over 90% of the total resistance to mass transfer. Heat flow from the shelf surface to the subliming ice is impeded by three barriers: the interface between the shelf surface and the bottom of the tray used to contain the vials, the interface between the tray and the vial bottom, and the ice between the bottom of the vial and the sublimation surface. Heat flow is described in terms of heat transfer coefficients, defined as the ratio of the heat flux to temperature difference, where the heat transfer coefficients are experimentally determined for each vial and tray of interest. Vial and tray heat transfer coefficients increase with increasing pressure and are quite sensitive to variations in degree of flatness of the vial or tray bottom. Steady-state transport theory is used to define six equations with eight variables where the equations contain mass transfer resistances and heat transfer coefficients which are determined from laboratory experiments. The variables are the sublimation rate and the pressures and temperatures throughout the system. Our procedure is to fix two of the variables (i.e., chamber pressure and shelf temperature) and solve, via a computer program, for the other six variables. Solutions are obtained for 0, 20, 40, 60, 80. and 100% completion of primary drying, thereby providing sublimation rate and the relevant temperatures and pressures as a function of time during the freeze-drying cycle defined by the input parameters chosen (i.e., the chamber pressure and shelf temperature profile with time). Thus, computer-generated freeze-drying cycles may be generated for any combination of product, container, and process parameters desired. Agreement between theoretical and experimental cycles is satisfactory. Use of this approach is demonstrated with the following applications: (1) a study of the effect of changes in chamber pressure and shelf temperature on drying time and product temperature: (2) the effect of shelf temperature variability and vial heat transfer coefficient variability on uniformity of drying: and (3) cycle optimization.

A Model for Predicting Spatially and Time Resolved Convective Heat Transfer in Bowl-in-Piston Combustion Chambers

Abstract: A new model for convective in-cylinder heat transfer has been developed which calculates heat transfer coefficients based on a description of the in-cylinder flow field. The combustion chamber volume is divided into three regions in which differential equations for angular momentum, turbulent kinetic energy and turbulent dissipation are solved. The resultant heat transfer coefficients are strongly spatially non-uniform, unlike those calculated from standard correlations, which assume spatial uniformity. When spatially averaged, the heat transfer coefficient is much more peaked near TDC of the compression stroke as compared to that predicted by standard correlations. This is due to the model's dependence on gas velocity and turbulence, both of which are amplified near TDC. The new model allows a more accurate calculation of the spatial distribution of the heat fluxes. This capability is essential for calculation of heat transfer and of component thermal loading and temperatures.

Particle Dynamics and Particle Heat and Mass Transfer in Thermal Plasmas. Part II. Particle Heat and Mass Transfer in Thermal Plasmas

Abstract: This paper is concerned with a review of heat and mass transfer between thermal plasmas and particulate matter. In this situation various effects which are not present in ordinary heat and mass transfer have to be considered, including unsteady conditions, modified convective heat transfer due to strongly varying plasma properties, radiation, internal conduction, particle shape, vaporization and evaporation, noncontinuum conditions, and particle charging. The results indicate that (i) convective heat transfer coefficients have to be modified due to strongly varying plasma properties; (ii) vaporization, defined as a mass transfer process corresponding to particle surface temperatures below the boiling point, describes a different particle heating history than that of the evaporation process which, however, is not a critical control mechanism for interphase mass transfer of particles injected into thermal plasmas; (iii) particle heat transfer under noncontinuum conditions is governed by individual contributions from the species in the plasma (electrons, ions, neutral species) and by particle charging effects.

Hydrodynamics of fluidization: Prediction of wall to bed heat transfer coefficients

Abstract: A computer model for a hot fluidized bed was developed. The large heat transfer coefficients characteristic of fluidized beds were computed without an enhancement of heat transfer by turbulence. They agreed with measurements reported by Ozkaynak and Chen (1980) within the accuracy of estimated thermal conductivity of solids.

Efficiency Parameters for Heat Transfer in Tube Banks

Abstract: Correlations for heat transfer and pressure drop for in-line and staggered banks of tubes dre suggested. They apply to Re from 1 to 2 × 106 and Pr from 0.7 to 104, and a wide range of relative transverse and longitudinal pitches. A technique of optimization of bank geometries is suggested on the basis of the authors’ experiments and the other published data. The suggested efficiency factor Eu Re3 may be useful for the interpretation of the available vast experimental data and for the evaluation of efficiency of heat transfer surfaces in further improvements of heat exchanger construction.

Flux formulation of hyperbolic heat conduction

Abstract: The development of the general flux formulation for heat conduction based on the modified Fourier’s law is presented. This new formulation produces a hyperbolic vector equation in heat flux which is more convenient to use for analysis in situations involving specified flux conditions than the standard temperature formulation. The recovery of the temperature distribution is obtained through integration of the energy conservation law with respect to time. The Green’s function approach is utilized to develop a general solution for hyperbolic heat conduction in a finite medium. The utility of the flux formulation and the unusual nature of heat conduction based on the hyperbolic formulation are demonstrated by developing analytical expressions for the heat flux and temperature distributions in a finite slab exposed to a pulsed surface heat flux.

Augmentation of Laminar Flow Heat Transfer in Tubes by Means of Wire Coil Inserts

Abstract: Etude experimentale de la chute de pression isotherme et du transfert de chaleur par convection vers une huile en ecoulement laminaire dans un tube lisse et sept tubes munis de bobines de fil metallique de differents diametres et placees de differentes manieres. Evaluation des performances de ces tubes a bobines

Effect of Wall Heat Conduction on Natural Convection Heat Transfer in a Square Enclosure

Abstract: This paper presents numerical and experimental results for buoyancy-induced flow in a two-dimensional, fluid-filled enclosure. Rectangular cavities formed by finite conductance walls of different void fractions and aspect ratios are considered. Parametric heat transfer calculations have been performed and results are presented and discussed. Local and average Nusselt numbers along the cavity walls are reported for a range of parameters of physical interest. The temperatures in the walls were measured with thermocouples, and the temperature distributions in the air-filled cavity were determined using a Mach-Zehnder interferometer. Good agreement has been obtained between the measured and the predicted temperatures in both the solid wall and in the fluid using the mathematical model. Wall heat conduction reduces the average temperature differences across the cavity, partially stabilizes the flow, and decreases natural convection heat transfer.

Heat Transfer Mechanism and Associated Turbulence Structure in the Near-Wall Region of a Turbulent Boundary Layer

Abstract: Heat transfer mechanism and associated turbulence structure in the near-wall region of a turbulent boundary layer have been investigated experimentally. With the aid of temperature-sensitive liquid crystal and hydrogen bubble method, the wall temperature fluctuation and the flow structure are visualized simultaneously to study directly the heat exchange between the fluid and the wall. The structure of the temperature field near the wall is further studied by the measurement of cross correlation of temperature fluctuations in the spanwise and streamwise directions. All of these experimental results show that the turbulent temperature field near the wall has a feature very much similar to the flow structure in the viscous wall region, e.g., the streamwise elongated structure with a quasiperiodicity in the spanwise direction, and are compatible with what is known about the streamwise sublayer vortices previously studied.

Unsteady heat transfer during laminar flame quenching

Abstract: Measurements were made of the unsteady heat transfer to a wall during the quenching of premixed, methane-air flames. One dimensional laminar flames were produced in a constant volume chamber and the heat transfer into the quenching surface was measured by means of a platinum thin film resistance thermometer. The experiments were performed at pressures near atmospheric over a range of equivalence ratios from 0.7 to 1.2. Predictions of the heat transfer were made with two numerical finite difference models, one with detailed kinetics and the other with single step kinetics. The main experimental results are: 1) the data are successfully correlated using the heat release rate of the flame prior to quenching; 2) the maximum heat flux is related to the quenching distance and thus it may be possible to use measurements of the quenching distance to predict the maximum heat flux. A comparison of the experimental results and the numerical calculations revealed that: 1) a single step reaction model predicts the heat transfer as well as a detailed kinetics model, to within 15% of the experimental results; 2) thermal diffusion and the chemical reaction rate of combustion are the dominant processes which determine the heat flux during quenching.