Jacob N. Chung

Evaporation of water into a laminar stream of air and superheated steam

Abstract A numerical investigation was conducted to study the evaporation of water into a laminar stream of air, humid air and superheated steam. Results were obtained both for variable properties and for constant properties using the one-third rule. It was shown that below a certain temperature of the free stream, named the inversion temperature, water evaporation decreases as the humidity of air increases; and above the inversion temperature, water evaporation increases as the humidity of air increases. The constant-property approximation using the one-third rule yields accurate results for the evaporation rate of water except when the free stream is mostly steam at high temperature.

Linear stability analysis of mixed-convection flow in a vertical pipe

A comprehensive numerical study on the linear stability of mixed-convection flow in a vertical pipe with constant heat flux is presented with particular emphasis on the instability mechanism and the Prandtl number effect. Three Prandtl numbers representative of different regimes in the Prandtl number spectrum are employed to simulate the stability characteristics of liquid mercury, water and oil. The results suggest that mixed-convection flow in a vertical pipe can become unstable at low Reynolds number and Rayleigh numbers irrespective of the Prandtl number, in contrast to the isothermal case. For water, the calculation predicts critical Rayleigh numbers of 80 and −120 for assisted and opposed flows, which agree very well with experimental values of Ra c = 76 and −118 (Scheele & Hanratty 1962). It is found that the first azimuthal mode is always the most unstable, which also agrees with the experimental observation that the unstable pattern is a double spiral flow. Scheele & Hanrattys speculation that the instability in assisted and opposed flows can be attributed to the appearance of inflection points and separation is true only for fluids with O (1) Prandtl number. Our study on the effect of the Prandtl number discloses that it plays an active role in buoyancy-assisted flow and is an indication of the viability of kinematic or thermal disturbances. It profoundly affects the stability of assisted flow and changes the instability mechanism as well. For assisted flow with Prandtl numbers less than 0.3, the thermal–shear instability is dominant. With Prandtl numbers higher than 0.3, the assisted-thermal–buoyant instability becomes responsible. In buoyancy-opposed flow, the effect of the Prandtl number is less significant since the flow is unstably stratified. There are three distinct instability mechanisms at work independent of the Prandtl number. The Rayleigh–Taylor instability is operative when the Reynolds number is extremely low. The opposed-thermal–buoyant instability takes over when the Reynolds number becomes higher. A still higher Reynolds number eventually leads the thermal–shear instability to dominate. While the thermal–buoyant instability is present in both assisted and opposed flows, the mechanism by which it destabilizes the flow is completely different.

Conjugate unsteady heat transfer from a spherical droplet at low Reynolds numbers

Abstract The phenomena of conjugate unsteady heat transfer from a spherical droplet or particle moving in a continuous fluid medium is numerically investigated. The energy equation is solved for a spherical droplet using the implicit, finite-difference method of alternating directions (ADI). In this study, the volumetric heat capacities of the two phases are of comparable magnitude but not necessarily equal to each other and the value of the thermal diffusivities of the two phases are set equal to each other. The range of Peclet numbers investigated are : 50⩽ Pe ⩽ 1000, with ratios of volumetric heat capacities, (interior to exterior) varying between 0.333 and 3.0. The velocities used in the convective terms are those corresponding to low Reynolds number flow. It was found that the dimensionless temperature profile asymptotically approaches a steady-state value that is independent of the initial profile in the droplet.

Unsteady mixed convection heat transfer from a solid sphere: The conjugate problem

Abstract Heat transfer associated with a spherical particle under simultaneous free and forced convection is numerically investigated using a combined Chebyshev Legendre spectral method. Both internal and external thermal resistances are taken into consideration by means of a conjugate model consisting of the full Navier -Stokes equations for external flow and the energy equations for both inside and outside the sphere. An influence matrix technique is employed to resolve the difficulties created by the lack of vorticity boundary conditions and to decouple the energy equations from interfacial couplings. Simulation results reveal that effects due to natural convection are most remarkable in the wake where the flow structure is changed. The overall Nusselt number and the drag coefficient show an increase or decrease in magnitude depending on whether gravity-induced flow aids or opposes the main flow. However, the change does not exceed 17% for the cases Gr/Re2 ⩽ 40 . When the buoyancy and the free stream are in the same direction, the effects are less pronounced than when they are in the opposite direction.

Terrestrial and microgravity boiling heat transfer in a dielectrophoretic force field

Abstract In order to maintain steady nucleate boiling in microgravity another force must be imposed on the boiling process to replace the buoyancy force. The objective of this study is to investigate the effectiveness of a static electric field for maintaining nucleate boiling in microgravity. Semi-transparent gold-film heaters are used to measure the instantaneous average heater surface temperature and to provide a bottom view of the boiling process. Three electrode geometries are designed with this heater: a diverging plate, a flat plate, and a pin electrode. Depending on the heat flux level and voltage strength, it was found that each of these electrodes is able to effectively move the boiling bubbles away from the vicinity of the heater surface in microgravity. Both flow visualization and measured heat transfer data are obtained to verify these results.

SIMULATION OF WEDGE-SHAPED PRODUCT DEHYDRATION USING MIXTURES OF SUPERHEATED STEAM AND AIR IN LAMINAR FLOW

A numerical model was developed to study the effectiveness of superheated steam, humid air, and dry air as dehydration media for wedge-shaped food products. Superheated steam had slightly higher surface friction loss than humid air and dry air except at lower free-stream temperatures. Superheated steam also exhibited higher heat transfer rates than the other fluids. The combined effect of these two characteristics was the existence of an inversion point in dehydration rate. The inversion point was the temperature at which evaporation rates for the fluids were approximately equal. The inversion points for different wedge angles were observed at approximately 275°C. The evaporation rates were greater for superheated steam than for dry air or humid air when the fluid temperature was above the inversion point. However, the evaporation rates were less for superheated steam than for dry air or humid air when the temperature was below the inversion point.

Unsteady conjugate heat transfer from a translating fluid sphere at moderate Reynolds numbers

Abstract The conjugate unsteady heat transfer between a translating droplet and its surrounding fluid at moderate Reynolds number is numerically investigated. The energy equation is solved by the ADI finite difference method with fluid motions inside and outside the droplet simulated by a series-truncation spectral method. The range of Reynolds numbers investigated is between 0 and 50. The ratios of viscosity and thermal conductivity between a droplet and its ambient flow range from 0 to 10 7 and 0.01 to 3, respectively. It was found that by increasing the Reynolds number, the predicted rate of heat transfer is significantly increased for fluid spheres as a result of increased fluid motions both inside and outside the droplet. On the other hand, the transfer rate for a solid sphere is much less sensitive to the Reynolds number than are the fluid spheres. For a gas bubble, any increase in the Reynolds number only increases the amplitude and frequency of the fluctuations in the Nusselt number and the steady-state Nusselt number is nearly independent of the Reynolds number.

Transient heat transfer to a fluid sphere suspended in an electric field

Abstract The unsteady heat transfer to a droplet suspended in an electric field is numerically investigated, for the case where the bulk of the resistance is in the droplet. It was found that, like the case of a translating droplet, there is a maximum steady-state Nusselt number. In this case, for a stationary drop in an electric field, the maximum steady Nusselt number was found to be about 30. An alternating direction implicit scheme was employed to integrate the energy equation. The range of interior Peclet numbers investigated, based on the maximum velocity in the droplet, was from 5 to 2000.

Unsteady conjugate heat transfer associated with a translating spherical droplet: a direct numerical simulation

Thermal behavior of a liquid drop moving in an incompressible fluid of infinite extent is investigated numerically. Dynamical equations describing the temporal evolution of the flow and temperature fields of dispersed and continuous phases are solved by a hybrid spectral scheme in conjunction with the influence matrix technique to resolve the lack of vorticity boundary conditions. Both Chebyshev and Legendre polynomials are employed to expand the flow variables and temperature in the radial and angular directions, respectively. With the aid of the Galerkin and collocation methods, together with the first-order backward Euler time differencing, the governing partial differential equations reduce to a nonlinear system of algebraic equations. Numerical results reveal a discrepancy between quasi-steady and fully transient analyses at high Peclet numbers.

Experimental investigation of condensation heat transfer in small arrays of PCM-filled spheres

Abstract Experimental investigations on the condensation heat and mass transfer between flowing steam and a small regularly packed bed of encapsulated spheres filled with PCM (phase-change material) are performed. The objectives of the research are to obtain transient transport characteristics of the dual-latent heat thermal storage system during the charging process. A special device is used to instrument the inside of the sphere. This device retards the sinking of the unmelted solid PCM toward the bottom of the spherical shell, thus reducing the strong dependence of the heat flux on tangential angle location. Instantaneous heat transfer rates are obtained for spheres of three different sizes (with single sphere and small packed bed arrangements). Tests performed to note Reynolds number and Stefan number dependencies of the two phase transfer and energy storage are also carried out. The device worked well, particularly for the larger spheres (6.35 and 7.62 cm diameter), which allowed the normalized thermal energy stored to be correlated by a single dimensionless time scale for all of the tests performed.