Laminar Forced Flow Research Articles

Forced convection mass/heat transfer coefficient at the surface of the rotor of the sucking and forcing regenerative exchanger

Experimental investigations of mean mass/heat transfer coefficients at the surface of two different curved short ducts of rotors of the sucking and forcing regenerative heat exchanger were made for forced laminar flow in static conditions. An electrolytic technique was employed. The results for two types of ducts are presented finally by correlations for j M Chilton-Colburn coefficient vs Reynolds number. The results for one of the ducts are consistent with the previous results of the author for the inner surface and developed flow in a straight annular duct with similar L/ D h .

Unsteady and steady mass transfer by laminar forced flow against a rotating disk

Unsteady and steady mass transfer by laminar forced flow against a rotating disk

Coupling of laminar flow heat transfer in a vertical circular tube with external free convection

The interactive heat transfer between laminar forced flow in a vertical circular tube and free convection induced in the ambient fluid is analyzed theoretically, taking into account two-dimensional thermal conduction in a tube wall of finite length and thickness. The Graetz solution for the internal forced flow, an analytical solution of the Laplace equation for the wall conduction and a numerical solution for the external free convection are combined so as to satisfy continuity of the temperature and heat flux at both internal and external interfaces. Experiments have been carried out for air—air systems with the conducting wall made of aluminum or ceramic in order to obtain temperature profiles. Theoretical predictions agree well with the experimental data. The validity of the conventional approximation of wall conduction only in the radial direction is discussed on the basis of the present results.

Mixed convection with flow reversal in the thermal entrance region of horizontal and vertical pipes

Numerical analyses have been conducted for combined free and forced laminar convection flow at low Péclet numbers in the thermal entrance region of horizontal and vertical pipes. Effects of both buoyancy force and axial conduction on the hydrodynamic and heat transfer characteristics are systematically investigated, and numerical results are extensively presented for velocity and temperature profiles, distributions of the Nusselt number and wall shear stress in both horizontal and vertical pipes. In a horizontal pipe, the secondary flow pattern has already developed at the beginning of heating and a reverse flow occurs near the pipe top as the axial conduction and buoyancy effects become large. Consequently, the local Nusselt number at the pipe top and then the circumferential average Nusselt number decrease with increasing secondary flow. The regime of reverse flow is clearly identified in the Pe− Ra coordinates. In a vertical pipe, flow reversals are observed at the pipe center in the heating case and near the wall in the cooling case at relatively high | Gr Re |. The regime of reverse flow is identified for both heating and cooling cases in the Pe−| Gr Re | coordinates.

A high-speed, rotating-disc metalorganic chemical vapor deposition system for the growth of (Hg,Cd)Te and related alloys

A high-speed, rotating-disc metalorganic chemical vapor deposition (MOCVD) system has been developed for the growth of (Hg,Cd)Te and related alloys. Traditional MOCVD reactors do not control density gradient driven convection currents generated by nonuniform gas heating. The rotating disc interacts dynamically with gas flow in a manner that can be used to control the effects of gas thermal expansion. When the sample pedestal is rotated at high speed, viscous drag on the rotating disc surface creates a subductive gas flow at the center of the reactor. The opposing effect of convective buoyancy created by non-uniform gas heating can be neutralized, establishing stable forced laminar flow through proper choice of disc rotation rate and other operating conditions. An important novel aspect of this system design individually separates introduction of the tellurium and cadmium precursors from the combined mercury/bulk hydrogen flow in a manner that results in independent control of the cadmium and tellurium concentrations across the entire area of the deposition plane. As a result, uniform chemical access to the growth surface is achieved and highly uniform alloy films are obtained. These are the first reported results using the high-speed, rotating-disc system for growth of (Hg,Cd)Te. Basic system design concepts and technical details of both system construction and operation are reported. The system has been used to successfully demonstrate reproducible growth of high quality (Hg,Cd)Te on multiple substrates. Operational parameters and experimental results are reported, including slice-to-slice and run-to-run uniformity data. CdTe, ZnTe and (Cd,Zn)Te buffer layers grown in-situ using this single MOCVD system have previously been reported; initial results for (Hg,Zn)Te, CdSe and ZnSe film growth are also reported; future reports will include other wide band gap (Cd,Zn)(Te,Se) alloys.

Experimental study of condensation of vapors in the presence of noncondensable gas on a short horizontal tube.

Measurements of the rates of condensation of a steam, methanol and benzene vapor in the presence of air on a short horizontal tube for wide ranges of inlet air concentrations and vapor flow rates were carried out. The observed vapor phase heat and diffusion fluxes were compared with results of the authors’ previous numerical solution to heat and mass transfer on a cylinder with diffusive surface mass suction and injection under laminar forced flow conditions.By considering the results of our previous numerical solution and the present experiments, a new method for simulation of condensation of vapors in the presence of noncondensable gas on a short horizontal tube was proposed and compared with the experimental data. The effect of noncondensable gas on the rate of condensation was also discussed.

Mixed convection in an inclined channel with a discrete heat source

The effects of laminar forced flow on buoyancy-induced natural convection cells throughout the regions of natural, mixed, and forced convection have been numerically investigated for a parallel planes channel with a discrete heat source. Emphasis is placed on the influence of the inlet flow velocity and the inclination angle of the channel, and the local buoyancy induced by the discrete source. The results indicate that the overall Nusselt number of the source strongly depends on the inclination angle (γ) in the natural and mixed convection regimes when γ > 45°. On the other hand, the changes in Nu and θ s.max are negligible when the channel is from 0 to 45°, i.e. there is no significant penalty in heat transfer due to the inclination of the channel up to γ = 45°. As Gr increases at a fixed Re, the entrainment of the air from the downstream exit is observed for the case of aiding flow.

Laminar mixed convection flow in a vertical tube

A computational study is conducted to quantify the buoyancy effects on laminar forced flow of air in a heated/cooled vertical tube. Buoyancy assisting and buoyancy opposing flow cases are considered for heating/cooling rates that provide either uniform wall temperature or uniform heat flux conditions. The nature of the observed effects of various heating/cooling rates and configurations on the velocity and temperature development, as well as on flow reversal, is explained. Critical buoyancy parameters that signal the onset of flow reversal, as well as axial locations where flow reversal first occurs, are determined in the study. The effects of the buoyancy parameter on friction factors, Nusselt numbers, and flow and thermal entry lengths are documented.

Transient freezing of liquids in forced laminar flow inside a parallel plate channel

A simple analytical approximative solution was given for calculating the time dependent development of the ice-layers at the cooled walls inside a parallel plate channel. By ignoring the effect of acceleration, resulting from converging ice-layers in the axial direction, an analytical solution for the variation of the ice-layer thickness with time and axial position could be obtained. The approximative solution was checked by numerical calculations and good agreement was found.

Wind-aided flame spread over charring and non-charrring solids: An experimental investigation

This paper presents the results of a detailed experimental investigation on laminar forced flow wind-aided flame spread over wood (with external radiation) and PMMA in the ceiling configuration. The speed of propagation of the pyrolysis front and flame front and the production rates of major chemical species are measured as a function of time. The objectives are to study the dependence of the flame and pyrolysis front speeds on the free stream velocity and the oxygen mass fraction and to infer the local fuel pyrolysis rates from the measured production rates of major chemical species. Several important conclusions are derived from this study: (i) In the configuration studied, the pyrolysis front and the flame front, for both wood and PMMA, were found to be much closer to each other than predicted by the theoretical models. This is also true for the pyrolysis front and the flame front speeds regardless of the free stream velocity and/or the oxygen mass fraction. (ii) The pyrolysis front and the flame front speeds for both wood and PMMA vary nearly linearly with the free stream velocity as predicted by the theoretical models. With oxygen mass fraction they vary as Y o ∞ 1.1 for wood and Y o ∞ 1.4 for PMMA. (iii) Species measurements show that the pyrolysis mass flux becomes almost constant with the downstream distance from the leading edge for both wood and PMMA. This is in disagreement with Emmons' steady boundary layer burning theory, where the mass flux decays as x −0.5 . Clearly, a steady state is not achieved in the entire burning zone. Nevertheless, the flame spread rate is predicted well by models that use Emmons' solution in the burning zone. This indicates that the flame spread rate depends primarily on local heating of the solid by the flame tip in the adjacent preheat zone.

Convection mixte dans un te convergent

In this paper, we study the heat and momentum transfer in a tee-branch in which two laminar forced flows interact. The effect of the wall heating on the flow structure is visualized and numerically analyzed. The numerical results show the influence of the Richardson number and they qualitatively agree with experimental investigations.

Solution of an elliptic inverse convection problem using a whole domain regularization technique

The inverse convection problem for developing laminar forced flow in a channel is investigated. The straight inversion process is found to be susceptible to inherent random errors introduced either by the numerical computations and/or experimental errors. In order to overcome the ill-posed behavior associated with inverse problems, a whole domain regularization scheme tailored for elliptic differential equations is employed in solving the governing energy equation. The methodology is found to be effective in curbing the ill-posed symptoms inherent in inverse problems. When a simple smoothing procedure is tested in conjunction with a straight inversion scheme, it fails to alleviate such symptoms and exhibits highly unstable results. Nomenclature matrix of coefficients, Eq. (11) dimensionless standard deviation of the bottom wall temperature, Eq. (20) matrix of constants, Eq. (14) Gaussian random error channel width, m

Combined convection and radiation in participating laminar flow inside a parallel-plate duct with flux boundary conditions

The interaction of radiation and forced convection in hydrodynamically developed, thermally developing steady laminar forced flow of a gray fluid between two infinite parallel plates with the prescribed heat flux boundary conditions is investigated. The implicit finite difference scheme is applied to solve the energy equation, while high order P N method is used to solve the radiation part of this problem. The effects of the conduction-to-radiation parameter, the single scattering albedo, the optical thickness, the surface reflectivity of the plates and the inlet temperature on the surface temperature and local Nusselt number are studied.

Frictional heating effect on heat transfer in forced convective flows

An analytical — experimental investigation into the effect of frictional heating on heat transfer during a forced convective laminar flow over a plate has been undertaken. The temperature distribution in laminar flow past a flat plate is obtained as the superposition of the separate distributions obtained for flow past a heated flat plate with frictional effects neglected and for flow past an adiabatic wall with the presence of frictional heat generation. For the case of Prandtl number around unity analytical calculations confirmed by experimental results show that frictional heating of the fluid adjacent to the wall results in fluid temperatures in excess of the wall temperature for values of Eckert number greater than about 2.

Analytical solutions to two-dimensional diffusion type problems in irregular geometries

A methodology, based on the ideas associated with the generalized integral transform technique, is presented for developing analytical solutions to sufficiently general two-dimensional (i.e. x and t space coordinates) diffusion type problems in arbitrarily shaped singly or doubly connected regions when the boundary contour in one of the space coordinates can be expressed in terms of the other. The model is also applicable for determining fully developed velocity distribution in arbitrarily shaped straight ducts. To illustrate the application, laminar forced flow in a right angle triangle duct is considered and the results are compared to the available exact solutions to assess the accuracy of the method.

Mass-transport aspects of pollutant removal at indoor surfaces

The mass-transport-limited rate of pollutant deposition onto indoor surfaces is examined in this paper. Transport of both particles and highly reactive gases through the boundary layer of air adjacent to a surface is analyzed for three model airflow conditions: (1) natural convection flow along room surfaces, (2) forced laminar flow parallel to room surfaces, and (3) homogeneous turbulence in the core of the room. Transport mechanisms considered include convective diffusion, thermophoresis, and gravitational sedimentation. The predicted mass-transport-limited deposition velocity, averaged over the surfaces of a room, varies from order 10 −6 to 10 −2 m s −1 over the range of pollutant diffusivities and particle sizes encountered. Theoretical predictions are in rough agreement with the limited experimental data that have been taken inside rooms. Results show that if buildings were designed and operated such that natural convection of forced laminar flow conditions prevailed with surface temperatures a few degrees K warmer than the room air, soiling of vertical surfaces due to deposition of soot particles in the size range 0.1–1 μm diameter could be greatly reduced.

Effects of anisotropic scattering of radiation in laminar forced convection in a parallel‐plate duct

AbstractEffects of anisotropic scattering to heat transfer in hydrodynamically‐developed, thermally‐developing steady laminar forced flow of a gray fluid between two infinite parallel plates are investigated for the case when the inlet temperature of the fluid is less than the wall temperature. An implicit finite difference scheme is applied to solve the energy equation, while the high order PN method is used to solve the radiation part of this problem. The effects of the anisotropy, the conduction‐to‐radiation parameter, the optical thickness, the reflectivity of the plates and the inlet temperature on the local Nusselt number are studied.

Combined heat and mass transfer in laminar forced convection channel flows

This study is to examine the effects of latent heat transfer associated with the liquid film vaporization on the heat transfer in the laminar forced convection channel flows. Major non-dimensional parameters identified are Pr, Sc, and φ. Results are specifically presented for an air-water system under various conditions. The effects of system temperatures on the heat and mass transfer are investigated in great detail. Results show that the evaporation of the liquid film has significant influences on laminar forced convection heat transfer.

Gas phase depletion and flow dynamics in horizontal MOCVD reactors

Growth rates of GaAs in the MOCVD process have been studied as a function of both lateral and axial position in horizontal reactor cells with rectangular cross-sections. A model to describe growth rates in laminar flow systems on the basis of concentration profiles under diffusion controlled conditions has been developed. The derivation of the growth rate equations includes the definition of an entrance length for the concentration profile to developed. In this region, growth rates appear to decrease with the 1/3 power of the axial position. Beyond this region, an exponential decrease is found. For low Rayleigh number conditions, the present experimental results show a very satisfactory agreement with the model without parameter fitting for both rectangular and tapered cells, and with both H 2 and N 2 as carrier gases. Theory also predicts that uniform deposition can be obtained over large areas in the flow direction for tapered cells, which has indeed been achieved experimentally. The influence of top-cooling in the present MOCVD system has been considered in more detail. From the experimental results, conclusions could be drawn concerning the flow characteristics. For low Rayleigh numbers (present study ≲ 700) it follows that growth rate distributions correspond with forced laminar flow characteristics. For relatively high Rayleigh numbers (present work 1700–2800), free convective effects with vortex formation are important. These conclusions are not specific for the present system, but apply to horizontal cold-wall reactors in general. On the basis of the present observations, recommendations for a cell design to obtain large area homogeneous deposition have been formulated. In addition, this work supports the conclusion that the final decomposition of trimethylgallium in the MOCVD process mainly takes place at the hot substrate and susceptor and not in the gas phase.

Measurements and Predictions of Laminar Mixed Convection Flow Adjacent to a Vertical Surface

Measurements and predictions of laminar mixed forced and free convection air flow adjacent to an isothermally heated vertical flat surface are reported. Local Nusselt numbers and the velocity and temperature distributions are presented for both the buoyancy assisting and opposing flow cases over the entire mixed convection regime, from the pure forced convection limit (buoyancy parameter ξ = Grx/Rex2 = 0) to the pure free convection limit (ξ = ∞). The measurements are in very good agreement with predictions and deviate from the pure forced and free convection regimes for buoyancy assisting flow in the region of 0.01 ≤ ξ ≤ 10 and for opposing flow in the region of 0.01<ξ< 0.2. The local Nusselt number increases for buoyancy assisting flow and decreases for opposing flow with increasing value of the buoyancy parameter. The mixed convection Nusselt numbers are larger than the corresponding pure forced and pure free convection limits for buoyancy assisting flow and are smaller than these limits for opposing flow. For buoyancy assisting flow, the velocity overshoot and wall shear stress increase, whereas the temperature decreases but the temperature gradient at the wall increases as the buoyancy parameter increases. The reverse trend is observed for the opposing flow. Flow reversal near the wall was detected for the buoyancy opposing flow case at a buoyancy parameter of about ξ = 0.20.