Showing papers on "Heat transfer coefficient published in 1992"

Heat transfer in rotating serpentine passages with trips normal to the flow

Abstract: Experiments were conducted to determine the effects of buoyancy and Coriolis forces on heat transfer in turbine blade internal coolant passages. The experiments were conducted with a large scale, multipass, heat transfer model with both radially inward and outward flow. Trip strips on the leading and trailing surfaces of the radial coolant passages were used to produce the rough walls. An analysis of the governing flow equations showed that four parameters influence the heat transfer in rotating passages: coolant-to-wall temperature ratio, Rossby number, Reynolds number, and radius-to-passage hydraulic diameter ratio. The first three of these four parameters were varied over ranges which are typical of advanced gas turbine engine operating conditions. Results were correlated and compared to previous results from stationary and rotating similar models with trip strips. The heat transfer coefficients on surfaces, where the heat increased with rotation and buoyancy, varied by as much as a factor of four. Maximum values of the heat transfer coefficients with high rotation were only slightly above the highest levels obtained with the smooth wall model. The heat transfer coefficients on surfaces, where the heat transfer decreased with rotation, varied by as much as a factor of three due to rotation and buoyancy. It was concluded that both Coriolis and buoyancy effects must be considered in turbine blade cooling designs with trip strips and that the effects of rotation were markedly different depending upon the flow direction.

Turbulent Heat Flux in the Upper Ocean Under Sea Ice

Abstract: Turbulence data from three Arctic drift station experiments demonstrate features of turbulent heat transfer in the oceanic boundary layer. Time series analysis of several w′T′ records shows that heat and momentum flux occur at nearly the same scales, typically by turbulent eddies of the order of 10–20 m in horizontal extent and a few meters in vertical extent. Probability distribution functions of w′T′ have large skewness and kurtosis, where the latter confirms that most of the flux occurs in intermittent “events” with positive and negative excursions an order of magnitude larger than the mean value. An estimate of the eddy heat diffusivity in the outer (Ekman) part of the boundary layer, based on measured heat flux and temperature gradient during a diurnal tidal cycle over the Yermak Plateau slope north of Fram Strait, agrees reasonably well with the eddy viscosity, with values as high as 0.15 m2 s−1. An analysis of measurements made near the ice-ocean interface at the three stations shows that heat flux increases with both temperature elevation above freezing and with friction velocity at the interface. It also reveals a surprising uniformity in parameters describing the heat and mass transfer: e.g., the thickness of the “transition sublayer” (from a modified version of the Yaglom-Kader theory) is about 10 cm at all three sites, despite nearly a fivefold difference in the under-ice roughness z0, which ranges from approximately 2 to 9 cm. A much simplified model for heat and mass transfer at the ice-ocean interface, suggested by the relative uniformity of the heat transfer coefficients at the three sites, is outlined.

Numerical calculation of wall‐to‐bed heat‐transfer coefficients in gas‐fluidized beds

Abstract: A computer model for a hot gas-fluidized bed has been developed. The theoretical description is based on a two-fluid model (TFM) approach in which both phases are considered to be continuous and fully interpenetrating. Local wall-to-bed heat-transfer coefficients have been calculated by the simultaneous solution of the TFM conservation of mass, momentum and thermal energy equations. Preliminary calculations suggest that the experimentally observed large wall-to-bed heat-transfer coefficients, frequently reported in literature, can be computed from the present hydrodynamic model with no turbulence. This implies that there is no need to explain these high transfer rates by additional heat transport mechanisms (by turbulence). The calculations clearly show the enhancement of the wall-to-bed heat-transfer process due to the bubble-induced bed-material refreshment along the heated wall. By providing detailed information on the local behavior of the wall-to-bed heat-transfer coefficients, the model distinguishes itself advantageously from previous theoretical models. Due to the vigorous solids circulation in the bubble wake, the local wall-to-bed heat-transfer coefficient is relatively large in the wake of the bubbles rising along a heated wall.

Turbulent Heat Transfer Augmentation and Friction in Periodic Fully Developed Channel Flows

Abstract: Measurements are presented of the distribution of average friction factors (f) as well as local and average ({ovr Nu}) heat transfer coefficients for fully developed channel flows with two rib-roughened opposite walls. The temperature measurements were made by using both a laser holographic interferometer and thermocouples. In addition, the reattachment length was determined by flow visualization. The Reynolds number (Re) was varied from 5.0 {times} 10{sup 3} to 5.4 {times} 10{sup 4}; the rib pitch-to-height ratios (Pi/H) were 10, 15, 20; and the rib height-to-hydraulic diameter ratios (H/De) were 0.063, 0.081, and 0.106. The detailed results allowed the peaks of heat transfer augmentation and the regions susceptible to hot spots to be located and allowed the relative contribution of the rib surface and the channel wall to the heat transfer augmentation to be determined. Moreover, relative to a smooth duct, the enhancement of both {ovr Nu} and f at various Re, Pi/H, and H/De was documented in detail. Furthermore, compact correlations in terms of Re, Pi/H, and H/De were developed for both {ovr Nu} and f.

Enhancement of Single-Phase Heat Transfer and Critical Heat Flux From an Ultra-High-Flux Simulated Microelectronic Heat Source to a Rectangular Impinging Jet of Dielectric Liquid

Abstract: Jet impingement is encountered in numerous applications demanding high heating or cooling fluxes. Examples include annealing of metal sheets and cooling of turbine blades, x-ray medical devices, laser weapons, and fusion blankets. The attractive heat transfer attributes of jet impingement have also stimulated research efforts on cooling of high-heat-flux microelectronic devices. These devices are fast approaching heat fluxes in excess of 100 W/cm[sup 2], which have to be dissipated using coolants that are both electrically and chemically compatible with electronic components. Unfortunately, fluids satisfying these requirements tend to possess poor transport properties, creating a need for significant enhancement in the heat transfer coefficient by such means as increased coolant flow rate and phase change. The cooling problem is compounded by a need to cool large arrays of heat sources in minimal volume, and to reduce the spacing between adjacent circuit boards. These requirements place severe constraints on the packaging of jet impingement cooling hardware.

Heat transfer in an electrically conducting fluid over a stretching surface

Abstract: An analysis is carried out to study the heat transfer characteristics in an electrically conducting fluid over a stretching sheet with variable wall temperature and internal heat generation or absorption. Two eases are studied, namely, (i) the sheet with prescribed surface temperature (PST case) and (ii) the sheet with prescribed wall heat flux (PHF case). The solutions for the temperature, the heat transfer characteristics and their asymptotic limits for large Prandtl number (σ) are obtained in terms of Kummer's and parabolic cylinder functions. It is shown that asymptotic limits are not possible for small Prandtl number; this is due to the solution changing by O(1) on length scale of 1/σ. For large Prandtl number, a boundary layer of width 1/σ at η = 0 and an internal layer of width ∝1/σ near the turning point are noticed.

Optimum Dimensions of Extended Surfaces Operating in a Convective Environment

Abstract: This article is devoted to the review of the literature on optimum dimensions of extended surfaces losing heat by pure convection to the surroundings. The review covers straight (longitudinal) fins, annular (radial) fins, and spines of different profile shapes. The optimum dimensions for each shape are given both in terms of the volume of the material as well as the heat dissipation. The effects of tip heat loss, variable heat transfer coefficient, internal heat generation, temperature dependent thermal conductivity, base convection, and primary surface thickness on the optimum dimensions are discussed. The optimization procedure is illustrated with several numerical examples. Areas of extended surface technology where further optimization studies are needed are identified. It is hoped that the article would serve the dual purpose of the state-of-the-art as well as a pedagogical review. 24 refs., 22 figs., 9 tabs.

A MODEL FOR HEATING of LIQUID‐PARTICLE MIXTURES IN A CONTINUOUS FLOW OHMIC HEATER

Abstract: Design of safe thermal processes in a continuous flow ohmic heater requires mathematical model development, since temperature monitoring of particles in continuous flow is not presently possible. A mathematical model previously developed and verified for a static heater was modified to predict temperatures of fluids and particles within a continuous heater with high particle concentration suspensions. Results for constant voltage simulations indicate that the presence of large populations of low electrical conductivity particles results in slow heating of the entire mixture rather than a single phase alone. In all these cases, particles tend to heat faster than the liquid. However, if isolated low-conductivity particles enter the system, the danger of underprocessing exists; hence particle conductivity is a critical control point. Simulations also indicate the role played by residence time distribution and liquid-particle heat transfer coefficients in the above cases.

Effect of Nozzle Configuration on Transport in the Stagnation Zone of Axisymmetric, Impinging Free-Surface Liquid Jets: Part 2—Local Heat Transfer

Abstract: This is the second of a two-part study on the flow structure and heat transfer characteristics of turbulent, free-surface liquid jets. Part 2 deals with the effect of selected nozzle configurations on the local heat transfer in the stagnation zone. Infrared techniques have been used to characterize the local heat transfer for the four nozzle configurations whose mean and turbulent flow structure was detailed in Part 1. The results show that for identical jet Reynolds numbers, significant differences exist in the magnitudes of the local Nusselt number for the nozzle types studied. Differences of approximately 40 percent were observed. Local heat transfer results reveal that for already turbulent jets, the mean radial velocity gradient appears to be more influential in determining the heat transfer than incremental changes in the level of turbulence (as measured by the radial component of the fluctuations). An empirical correlation of the experimental data supports this conclusion, and reveals that the stagnation Nusselt number is affected independently by the jet Reynolds number and the dimensionless mean radial velocity gradient. 21 refs., 6 figs.

Natural convection heat transfer in rectangular cavities partially heated from below

Abstract: Natural convection in an enclosed cavity with localized heating from below has been investigated by a finite difference procedure. The upper surface is cooled at a constant temperature and a portion of the bottom surface is isothermally heated while the rest of the bottom surface and the vertical walls are adiabatic. Parameters of the problem are the cavity aspect ratio (A = 1 and 2), dimensionless length (B = 0.06 to 1.0) and position of the heat source with respect to the vertical symmetry line of the cavity (e = -0.6 to 0.7), the Prandtl number and the Rayleigh number (Ra = 0 to 5 x 10 6). The effects of the thermophysical and geometrical parameters on the fluid flow and temperature fields have been studied. The existence of multiple steady-state solutions and the oscillatory behavior for a given set of the governing parameters are demonstrated. Nomenclature A = aspect ratio, L'lH' B = dimensionless length of heat source, t'/L' g = acceleration due to gravity, m/s2 H' = cavity height, m h = local heat transfer coefficient, W/m2-K h = average heat transfer coefficient, W/m2-K k = thermal conductivity of fluid, W/m-K L' = cavity width, m €' = length of heat source, m m, n = wave numbers of initial disturbance, Eq. (15) Nu = Nusselt number based on cavity height, hH'/K Pr = Prandtl number,'via p'_ = pressure, Pa

The Effect of Plate Spacing on Free Convection Between Heated Parallel Plates

Abstract: In the past, considerable attention has been given to free convection between heated vertical parallel plates. This problem is considerable interest to engineers because of its application to electronic equipment cooling and solar energy. Some attempts have been made to optimize the spacing between parallel plates in the past. Bodoia and Osterle analytically derived a criterion for an optimum plate spacing for which the heat dissipation is maximum. The objective of this paper is to predict the optimum plate spacing for single channels by using the governing equations for a continuous system model. No heat transfer coefficient known a priori will be used in these calculations, but will be calculated as part of the solution.

Increased surface temperature effects on wall heat transfer during unsteady flame quenching

Abstract: The coupling of thermal and chemical processes is significant during laminar flame quenching. A direct thermal response of a quenching flame is the heat transfer to the wall. The unsteady heat transfer during premixed laminar flame quenching was measured in a constant volume chamber over a range of wall temperatures from 298 K to 423 K and a range of equivalence ratios from 0.8 For all of the measurements, the maximum heat flux could be correlated to the heat release rate in the steady flame prior to quenching. The fraction of the heat release rate attributed to heat transfer was independent of the equivalence ratio but dependent on the wall temperature. The experimental results were independent of buoyancy and catalytic effects and of whether the wall was locally heated or the entire bomb was globally heated. Calculations of the heat transfer were made for both one dimensional and two dimensional flame quenching using finite difference methods with chemistry specified as a single reaction step. Comparisons of the numerical results with the experimental data emphasized the sensitivity of the heat flux to the specifications of the reaction mechanism parameters. In particular, it was found that the single step mechanism and simplified chemical transport models could not predict the dependence of the wall heat transfer on the wall temperature. It was concluded that during quenching low activation energy recombination reactions may contribute significantly to the deviation between the single step thermal reaction mechanism and the observed experimental results.

Experimental investigation of film cooling effectiveness for slots of various exit geometries

Abstract: A parametric study was conducted to experimentally determine the effects slot exit geometries have on film effectiveness (ijy) for several injection angles (a = 0.0, 5.0, 8.5, 11.5, and 15.0 deg). Such slot geometries are typically used on the pressure side trailing edge of jet engine turbine airfoils, providing cooling film to protect the trailing edge, which is often a life limiting area. Four different slot lip thickness to height ratios (tls = 0.5, 0.75, 1.0, and 1.25) and three different slot width to height ratios (w/s = 2, 5, and 17) were tested over a blowing ratio (M = (pU)c/(pU)h) range of 0 to 1.3. All geometries were tested at a constant density ratio (pc/ph) of 1.4. Slot surface film effectiveness measurements were made over a range of downstream surface distance to slot height ratios (x/s) of 0 to 15. Five different density ratios (pclph = 1.2, 1.3, 1.4, 1.5, and 1.6), spanning the typical engine operating range, were tested for one geometry to determine the effect of density ratio on film effectiveness. Correlations are shown for film effectiveness (rjf) in terms of the nondimensionalized downstream distance (x/Ms). The results show that: a) ij/is highly sensitive to tls, but not significantly sensitive to either w/s or pclph and b) an optimum injection angle equal to 8.5 deg exists for xlMs values less than 60.

Heat Transfer With Very High Free-Stream Turbulence: Part I—Experimental Data

Abstract: Boundary layer heat transfer with very high freestream turbulence is investigated. The problem is studied experimentally by placing a constant-temperature heat transfer surface at various locations in the margin of a turbulent free jet and measuring both the surface heat transfer rate and the turbulence in the freestream. Freestream turbulent fluctuations 20 to 60 percent relative to the mean velocity augment heat transfer 1.8 to 4 times that which would be predicted locally using accepted correlations for turbulent boundary layers at the same Reynolds number. The correlations of Simonich and Bradshaw (1989), Pedisius et al. (1983), and Blair (1983) each fail to describe the present data. For flows over flat surfaces in air with very high freestream turbulence, greater than 0.2, u-prime determines h. A new heat transfer parameter, St-prime, characterizes turbulent boundary layer heat transfer with freestream turbulence on the domain 0-0.65 to within +/- 15 percent for high Reynolds number flows with uniform thermal boundary conditions.

Thermal Energy Analysis in Reciprocating Hermetic Compressors

Abstract: In this paper the thermal energy analysis of a reciprocating hermetic compressor is performed using a computational program The simulation model employed in the program is based on energy balances For the refrigerant gas inside the compressor cylinder use was made of the first law of thermodynamics including time variations of the mass and energy fluxes The required temperatures at the suction chamber, cylinder walls, discharge chamber, discharge muffler, compressor shell, and ambient inside the compressor shell are obtained from steady state energy balances at various locations within the compressor Effective overall heat transfer coefficients were determined experimentally, except for the heat transfer between the refrigerant and the cylinder walls which was obtained from existing correlations A companion simulation program which represents the compressor working features was used to calculate the mass fluxes at the suction, discharge, and the leakage flux Simulation results are presented for a small compressor and compared with experimental results Good agreement prevails indicating that the major effects affecting the thermal performance of the compressor have been considered by the proposed model INTRODUCTION There are several reports in the literature concerning numerical models to predict the performance of hermetic refrigeration compressors Those models require thermodynamic relationships to describe the behavior of the gas inside the cylinder These relationships can be obtained either through a polytropic transformation or through an energy balance A furth-er aspect to be considered is the heat transfer to and from the refrigerant as it passes through the compressor The models described in the literature differ mainly on how the two aforementioned issues are addressed On the next paragraphs some of the major works done on compressor modelling will be reviewed Qvale eta/ [l] indicated some areas where research should be done to improve the current knowledge on compressor modelling Heat transfer on the suction and discharge lines, on the valves, and between the gas and the cylinder walls are the most important points mentioned by the authors According to [l] the numerical models dealing with the cylinder of hermetic compressors have frequently employed the assumption of perfect gas in a polytropic process The exponent of the polytropic equation is usually adjusted to fit experimental results In this regard the polytropic index incorporates the combined effect of the heat transfer between gas and cylinder, friction, and deviations from the perfect gas behavior Therefore, the influence of these effects separately cannot be detected Karl! [2] investigated through the first law of thermodynamics the process described by a real gas inside the cylinder He considered the cylinder as a close system undergoing exchange of heat and work with the surroundings Prakash and Singh [3] also employed the first law of thermodynamics but assumed perfect gas behavior The heat transfer between the cylinder walls and the gas was predicted using the correlation given by Adair et a/ [4]· Rottger and Kruse [5] verified through their model that for the compressor performance it is important to use the equation of state for real gas but for the valve performance suffices to use the perfect

Design And Technology Of Heat Pipes For Cooling And Heat Exchange

Abstract: Introduction HEAT PIPE CHARACTERISTICS The Role of Capillarity in Heat Transport Pressure and Temperature Distributions Heat Transport Limits Heat Pipe Startup References HEAT PIPE TECHNOLOGY Heat Transport Fluids Containment and Wick Materials Wick Characteristics Heat Pipe Fabrication Environmental Influences on Heat Pipe Operation Heat Pipe Systems References FLUID FLOW IN A HEAT PIPE The Nature of the Flow Process General Pressure Drop Formulation Frictional Pressure Drop for Constant Surface Mass Flux Frictional Pressure Drop for Nonconstant Surface Mass Flux Hydraulic Diameter Frictional Pressure Drop in a Porous Flow Passage Frictional Pressure Drop for Convective Cooling References HEAT TRANSPORT LIMITS Capillary Pumping Limit Sonic Limit Entrainment Limit Boiling Limit Heat Pipe Operational Boundaries Comparison of Calculated and Experimental Heat Transport Limits References HEAT PIPE DESIGN: STEADY STATE Design Criteria and Constraints Heat Pipe Area-Temperature Relations Heat Pipe Internal Dimensions Structural Considerations Additional Design Topics Heat Pipe Exchangeers References HEAT PIPE DESIGN: TRANSIENT BEHAVIOR Heat Pipe Startup Features of the Transient Model Transient Equations Calculational Procedure Assessment of Heat Pipe Startup References DESIGN EXAMPLES Heat Pipe Space Radiator Transient Analysis of the Heat Pipe Space Radiator References Appendices

Prediction of Heat Transfer and Friction for the Louver Fin Geometry

Abstract: This paper is concerned with prediction of the air-side heat transfer coefficient of the louver fin geometry used in automotive radiators. An analytical model was developed to predict the heat transfer coefficient and friction factor of the louver fin geometry. The model is based on boundary layer and channel flow equations, and accounts for the[open quote] flow efficiency[close quotes] in the array, as previously reported by Webb and Trauger. The model has no empirical constants. The model allows independent specifications of all of the geometric parameters of the touver fin. This includes the number of louvers over the flow depth, the louver width and length, and the louver angle. The model was validated by predicting the heat transfer coefficient antifriction factor of 32 louver arrays tested by Davenport, which spanned hydraulic diameter based Reynolds numbers of 300-2800. At the highest Reynolds number, all of the heat transfer coefficients were predicted within a maximum error of [minus]14 /+ 25 percent, and a mean error of +/- percent. The high Reynolds number friction factors were predicted with a maximum error [minus]22 / + 26 percent, with a mean error of +/- 8 percent. The error ratios were slightly higher at the lowestmore » Reynolds numbers. 11 refs., 14 figs., 2 tabs.« less

Optimized offset strip fin for use in compact heat exchangers

Abstract: An offset strip fin for use in compact automotive heat exchangers is disclosed. The offset strip fin has multiple transverse rows of corrugations extending in the axial direction wherein the corrugations in adjacent rows overlap in order that the oil boundary layer is continually re-started. The fin dimensions have been optimized in order to achieve superior ratio of heat transfer to pressure drop along the axial direction. In one aspect, . .an.!. .Iadd.a .Iaddend.compact concentric tube heat exchanger has an offset set strip tin located in an annular fluid flow passageway located between a pair of concentric tubes. The preferred range of lanced lengths is determined to be between 0.035" to 0.075" for periodically developed flow. Maintaining the lanced length in the regime of periodically developed flow is advantageous in that it gives a higher heat transfer coefficient than is achievable with fully developed flow. This also provides the added advantage that variations in the shape of the flow passages from the rectangular do not impact negatively on the heat transfer. .Iadd.The corrugations have top portions and bottom portions of the same width. The height of the corrugations is greater than this width. .Iaddend.