Showing papers on "Nusselt number published in 1990"

Effect of a centered conducting body on natural convection heat transfer in an enclosure

Abstract: The effect of a centered, square, heat-conducting body on natural convection in a vertical square enclosure was examined numerically. The analysis reveals that the fluid flow and heat transfer processes are governed by the Rayleigh and Prandtl numbers, the dimensionless body size, and the ratio of the thermal conductivity of the body to that of the fluid. For Pr = 0.71 and relatively wide ranges of the other parameters, results are reported in terms of streamlines, isotherms, and the overall heat transfer across the enclosure as described by the Nusselt number. Heat transfer across the enclosure, in comparison to that in the absence of a body, may be enhanced (reduced) by a body with a thermal conductivity ratio less (greater) than unity. Furthermore, the heat transfer may attain a minimum as the body size is increased. These and other findings are justified through a careful examination of the local heat and fluid flow phenomena.

Film evaporation from a micro-grooved surface―an approximate heat transfer model and its comparison with experimental data

Abstract: An analytical model is presented that can be used to predict the heat-transfer characteristics of film evaporation on a microgroove surface. The model assumes that the liquid flow along a "V"-shaped groove channel is driven primarily by the capillary pressure difference due to the receding of the meniscus toward the apex of the groove, and the flow up the groove side wall is driven by the disjoining pressure difference. It also assumes that conduction across the thin liquid film is the dominant mechanism of heat transfer. A correlation between the Nusselt number and a nondimensional parameter ¥ is developed from this model which relates the heat transfer for the microgroove surface to the fluid properties, groove geometry, and the constants for the disjoining pressure relation. The results of a limited experimental study of the heat transfer during vaporization of a liquid coolant on a microgroove surface are also reported. Film-evaporation transfer coefficients inferred from these experiments are found to correlate fairly well in terms of Nusselt number and ¥ parameter format developed in the model. The results of this study suggest that disjoining pressure differences may play a central role in evaporation processes in microgroove channels.

Analysis of Surface Enhancement by a Porous Substrate

Abstract: Convective flow and heat transfer through a composite porous/fluid system have been studied numerically. The composite medium consists of a fluid layer overlaying a porous substrate, which is attached to the surface of the plate. The numerical simulations focus primarily on flows that have the boundary layer characteristics. However, the boundary layer approximation was not used. A general flow model that accounts for the effects of the impermeable boundary and inertia is used to describe the flow inside the porous region. Several important characteristics of the flow and temperature fields in the composite layer are reported. The dependence of these characteristics on the governing parameters such as the Darcy number, the inertia parameter, the Prandtl number, and the ratio of the conductivity of the porous material to that of the fluid is also documented. The results of this investigation point out a number of interesting practical applications such as in frictional drag reduction, and heat transfer retardation or enhancement of an external boundary.

Flow reversal and heat transfer of fully developed mixed convection in vertical channels

Abstract: The present analysis is concerned with flow reversal phenomena and heat transfer characteristics of the fully developed laminar combined free and forced convection in the heated vertical channels. Three fundamental combinations of thermal boundary conditions on the respective wall surface (namely isoflux-isoflux, isofluxisothermal, and isothermal-isothermal) are considered separately so as to investigate extensively their distinct influence on the flow pattern. Results of the velocity distribution and temperature distribution as well as the Nusselt number in terms of bulk mean temperature are carried out. Based on the analytical solutions obtained, flow reversal adjacent to the relatively colder wall is found to exist within the channel as Re/Gr is below than a threshold value, which is related to the thermal boundary conditions. Parameter zones for the occurrence of reversed flow are presented. Comparisons and verification are made using the existing numerical solutions at locations far downstream of developing flow.

Effect of entrainment on the heat transfer to a heated circular air jet impinging on a flat surface

Abstract: An experimental investigation is described that characterizes the convective heat transfer of a heated circular air jet impinging on a flat surface. The radial distributions of the recovery factor, the effectiveness, and the local heat transfer coefficient are presented. The recovery factor and the effectiveness depend on the spacing from jet exit to the impingement plate, but do not depend on jet Reynolds number. The effectiveness does not depend on the temperature difference between the jet and the ambient. A correlation is obtained for the effectiveness. The heat transfer coefficient is independent of the temperature difference between the jet and the ambient if it is defined with the difference between the heated wall temperature and the adiabatic wall temperature.

Temperature and velocity field regimes of convective motions in a rotating plane fluid layer

Abstract: The paper is a continuation of work published in Boubnov & Golitsyn (1986). We present new measurements of the temperature and velocity field patterns and their statistical characteristics. This allows us to classify regimes of convection in a plane rotating horizontal fluid layer in terms of Rayleigh and Taylor numbers. Within the irregular regimes geostrophic convection is found for which the Rossby number is much less than unity.In the regular regimes the mean temperature profiles are linear with height in the bulk of the fluid, the gradient being dependent mainly on rotation rate Ω and fluid depth h. These together with some dimensional arguments lead to the heat transfer relationship Nu ∝ Ra3 Ta−2 between Nusselt, Rayleigh and Taylor numbers. Experimental results by Rossby (1969) and theoretical work by Chan (1974) and Riahi (1977) suggested this dependence. The dependence on ωτ of the temperature power spectrum normalized by the variance was found to be universal at higher frequencies for all irregular convective motions, where τ is the timescale of the thermal boundary layer for cases with a small influence of rotation and with τ about three times larger (in numerical coefficient) for geostrophic convection. For irregular geostrophic regimes it is found that the temperature variance depends on rotation rate and heat flux, and is inversely proportional to the buoyancy parameter.Horizontal and vertical components of the velocity fields were measured for regular as well as irregular regimes, confirming, especially for geostrophic convection, the theoretical results by Golitsyn (1980). In conclusion some geophysical applications are briefly mentioned.

High-Rayleigh-number convection in a horizontal enclosure

Abstract: A review of the literature on natural convection in a horizontal layer heated from below shows the need for reliable data at high Rayleigh number (Ra) to determine the asymptotic Nusselt number (Nu) variation with Rayleigh number. The present study expands the data base by the use of an electrochemical mass transfer technique to determine the asymptotic dependence of the Sherwood number (Sh) on Ra at high Schmidt number (Sc). The results of the present study give Sh = 0.0659 for Sc ≈ 2750, 3 × 109 < Ra < 5 × 1012. Using the heat-mass transfer analogy, this indicates the high Prandtl number variation of Nu with Ra.

Heat transfer to a micropolar fluid from a non-isothermal stretching sheet with suction and blowing

Abstract: The heat transfer from a stretching sheet to a micropolar fluid is analyzed using the theory of micropolar fluids formulated by Eringen. The governing equations for momentum, angular momentum and energy have been solved numerically. Numerical data for the friction factor and Nusselt number has been tabulated for a range of Prandtl numbers. Surface mass transfer rate and the power law constant for the wall temperature have considerable influence on the heat transfer mechanism.

Influence of High Mainstream Turbulence on Leading Edge Film Cooling Heat Transfer

Abstract: The influence of high mainstream turbulence on leading edge film effectiveness and heat transfer coefficient was studied. High mainstream turbulence was produced by a passive grid and a jet grid. Experiments were performed using a blunt body with a semicylinder leading edge with a flat afterbody. The mainstream Reynolds number based on leading edge diameter was about 100,000. Spanwise and streamwise distributions of film effectiveness and heat transfer coefficient in the leading edge and on the flat sidewall were obtained for three blowing ratios, through rows of holes located at ± 15 and ± 40 deg from stagnation

An Experimental Investigation of Heat Transfer in an Orthogonally Rotating Channel Roughened With 45 Degree Criss-Cross Ribs on Two Opposite Walls

Abstract: Turbine blade cooling is imperative in advanced aircraft engines. The extremely hot gases that operate within the turbine section require turbine blades to be cooled by a complex cooling circuit. This cooling arrangement increases engine efficiency and ensures blade materials a longer creep life. One principle aspect of the circuit involves serpentine internal cooling passes throughout the core of the blade. Roughening the inside surfaces of these cooling passages with turbulence promoters provides enhanced heat transfer rates from the surface. The purpose of this investigation was to study the effect of rotation, aspect ratio, and turbulator roughness on heat transfer in these rib-roughened passages. The investigation was performed in an orthogonally rotating setup to simulate the actual rotation of the cooling passages. Single-pass channels, roughened on two opposite walls, with turbulators positioned at 45 deg angle to the flow, in a criss-cross arrangement, were studied throughout this experiment. The ribs were arranged such that their pitch-to-height ratio remained at a constant value of 10. An aspect ratio of unity was investigated under three different rib blockage ratios (turbulator height/channel hydraulic diameter) of 0.1333, 0.25, and 0.3333. A channel with an aspect ratio of 2 was also investigated for a blockage ratio of 0.25. Air was flown radially outward over a Reynolds number range of 15,000 to 50,000. The rotation number was varied from 0 to 0.3. Stationary and rotating cases of identical geometries were compared. Results indicated that rotational effects are more pronounced in turbulated passages of high aspect and low blockage ratios for which a steady increase in heat transfer coefficient is observed on the trailing side as rotation number increases while the heat transfer coefficient on the leading side shows a steady decrease with rotation number. However, the all-smooth-wall classical pattern of heat transfer coefficient variation on the leading and trailing sides is not followed for smaller aspect ratios and high blockage ratios when the relative artificial roughness is high.

Turbulent thermal convection in a finite domain: Part II. Numerical results

Abstract: A pseudospectral method is used to solve the Boussinesq equations for a fully inhomogeneous turbulent flow. The numerical data are analyzed using the empirical eigenfunction technique. As a result of the underlying inhomogeneity of the flow, the eigenfunctions (structures) are inhomogeneous in all three directions. This is the first instance in which fully three‐dimensional empirical eigenfunctions have been calculated. The generated basis set is extremely efficient at depicting the flow. The first eigenfunction captures almost 60% of the average energy. The eigenfunctions are an optimal basis for capturing the energy of the flow and more than 95% of the energy is captured by the first 100 eigenfunctions. Ten classes of eigenfunctions are present and examples of each are shown. The average Nusselt number for the bounded geometry is found to be lower than that for a correspondong homogeneous case and the physics causing this decrease is analyzed and discussed.

Numerical simulations of soft and hard turbulence: Preliminary results for two-dimensional convection

Abstract: Results on the transition from soft to hard turbulence in simulations of two-dimensional Boussinesq convection are reported. The computed probability densities for temperature fluctuations are exponential in form in both soft and hard turbulence, unlike what is observed in experiments. In contrast, a change is obtained in the Nusselt number scaling on Rayleigh number in good agreement with the three-dimensional experiments.

Interaction between two vaporizing droplets in an intermediate Reynolds number flow

Abstract: The dynamic properties of two vaporizing droplets moving in tandem and interacting through hydrodynamic forces are presented for the intermediate Reynolds number range of droplet motion [Re=O(100)]. The problem is relevant for dense spray applications. Depending upon certain initial parameters, the droplets can collide or separate. The behaviors of the droplets are influenced by the mass and heat transport associated with the evaporation of droplets in air. Droplet‐drag coefficient, Nusselt number, droplet mass, droplet Reynolds number, and droplet spacing histories are reported for a limited number of cases following the introduction of two droplets of different radii, a’1,0 and a’2,0, at a given initial spacing, a’0, into a hot, convective fluid, which is characterized by uniform free‐stream conditions. A finite‐difference solution of the coupled two‐phase, unsteady Navier–Stokes equations in cylindrical coordinates on a body‐fitted coordinate system is adjusted continually to accommodate the changing b...

Numerical prediction of convective heat transfer in self-sustained oscillatory flows

Abstract: Numerical investigations of the flow pattern and heat transfer enhancement in supercritical grooved-channel and communicating-channels flows are presented. For Reynolds numbers above the critical one, Rc =0(100), these flows exhibit laminar self-sustained oscillations at the plane channel Tollmien-Schlichting frequency. These ordered, very well-mixed flows require significantly less pumping power than the random fluctuating turbulent flows to achieve the same transport rates. Comparing different heat transfer augmentation schemes in grooved channels, it is shown that the best enhancement system regarding minimum power dissipation corresponds to passive flow modulation in the range of low Nusselt numbers. However, spontaneous supercritical flow destabilization becomes competitive as the Nusselt number is increased. It is found that on an equal pumping power basis, the heat transfer in communicating channels flows is up to 300% higher than the one in flat channel flow.

Condensation heat transfer of binary refrigerant mixtures of R22 and R114 inside a horizontal tube with internal spiral grooves

Abstract: An experimental study of the condensation of pure and mixed refrigerants of R22 and R114 inside a spirally grooved horizontal copper tube has been carried out. A double-tube counterflow condenser in the pressure range 3–21 bar and at a mass flow-rate 26–70 kg h −1 was used. The axial distributions of refrigerant, tube wall and cooling water temperatures, wall heat flux density and vapour quality are shown graphically. The variation of tube wall temperature around the circumference of the tube is also shown. The local Nusselt number depends on the molar fraction, whereas the average Nusselt number can be correlated by an equation which is modified from a previously established equation for pure refrigerants inside a horizontal smooth tube. The frictional pressure drop evaluated is correlated well by the Lockhart-Martinelli parameters and is independent of the concentration of the mixture.

Film Cooling in the Presence of Mainstream Pressure Gradients

Abstract: Film-cooling in the presence of mainstream pressure gradients typical of gas turbines has been studied experimentally on a flat plate This paper describes, measurements of the spanwise averaged effectiveness and heat transfer coefficient for an inclined slot and a single row of holes in the presence of favourable, zero and adverse pressure gradients. Acceleration parameters of K = 2.62×10−6 and - 0.22 × 10−6 were achieved at the point of injection where the freestream unit Reynolds number was held constant at Re/m = 2.7 × 107. The flow was accelerated to high Mach number and results are analysed using a superposition model of film-cooling which included the effects of viscous energy dissipation. The experimental results show the effects of pressure gradient differ between the geometries and a discussion of these results is included. The unblown turbulent boundary layer with pressure gradient were also studied. Experiments were performed using the Isentropic Light Piston Tunnel, a transient facility which enables conditions representative of those in the engine to be attained.Copyright © 1990 by ASME

Parametric and Numerical Study of Fully-Developed Flow and Heat Transfer in Rotating Rectangular Ducts

Abstract: This work is concerned with fully-developed constant-density turbulent flow through rectangular straight ducts rotating in an orthogonal mode. Ducts of both square and 2:1 aspect ratio cross-sections have been examined. For the square duct, predictions have been performed for Reynolds numbers of 33,500 and 97,000 and for the 2:1 aspect ratio duct the computations were carried out for a Reynolds number of 33,500. Values of the inverse Rossby number (Ro = ΩD/Wb) ranged from 0.005 to 0.2. Except in the immediate vicinity of the wall, the standard high-Reynolds-number version of the k-e model is used to account for the effects of turbulence. Across the near-wall sublayer the damping of turbulence is modelled through a low-Reynolds-number one-equation model.Low rotational speeds cause the formation of a pair of symmetric streamwise vortices. At higher rotational speeds, flow instabilities on the pressure side lead to transition to a more complex four-vortex structure. The transition point depends on both the cross-sectional geometry and the flow Reynolds number. Moreover, over a range of Rossby number, either two- or four-vortex solutions are possible depending upon initial conditions.The rotation leads to significant differences between the values of friction factor and Nusselt number on the suction and pressure surfaces of the duct. The degree of heat transfer augmentation on the pressure side is found to depend on the Reynolds number as well as on Rossby number. In contrast, heat-transfer attenuation on the suction side is only Rossby-number dependent.Copyright © 1990 by ASME