Jung-Yang San

Impingement cooling of a confined circular air jet

A measurement of the local Nusselt number of a confined circular air jet vertically impinging on a flat plate is performed. The jet flow, after impingement, is constrained to exit in two opposite directions. Part of the impinging surface is maintained at a constant heat flux condition and the rest is adiabatic. The constant surface heat flux condition is modeled by conducting electricity through a thin stainless foil with a thickness of 0.01 mm. The heating foil was given constant heat flux values of 500, 1000, 1500 and 2000 W m−2. Four diameters of the impinging jet 3, 4, 6 and 9 mm are considered individually. The jet Reynolds number is in the range of 30,000-67,000. The Hd is 2.0. The recirculation and mixing effect on the heat transfer is investigated by varying the jet diameter, surface heat flux, Reynolds number and surface heating width. The correlations of the local Nusselt number and the jet-to-adiabatic wall temperature difference are obtained.

Optimum jet-to-jet spacing of heat transfer for staggered arrays of impinging air jets

Abstract The effect of jet-to-jet spacing on the local Nusselt number for confined circular air jets vertically impinging on a flat plate is investigated. Five jets in equilaterally staggered arrays are considered. The surface heat flux on the plate is 1500 W m −2 . The jet diameter is 3 mm. Three different jet Reynolds numbers (Re= 10,000, 20,000 and 30,000) are individually considered. The influence of jet-to-jet spacing, Re and jet height on the stagnation Nusselt number of the center jet is experimentally investigated. An optimum ratio of jet-to-jet spacing to jet diameter, (s/d)opt, is obtained. The existence of the (s/d)opt is attributed to jet interference before impingement and/or formation of jet fountain between two adjacent jets. The stagnation Nusselt number is correlated as a function of Re, s/d and H/d.

Entropy Generation in Convective Heat Transfer and Isothermal Convective Mass Transfer

The irreversible generation of entropy for two limiting cases of combined forced-convection heat and mass transfer in a two-dimensional channel are investigated. First, convective heat transfer in a channel with either constant heat flux or constant surface temperature boundary conditions are considered for laminar and turbulent flow. The entropy generation is minimized to yield expressions for optimum plate spacing and optimum Reynolds numbers for both boundary conditions and flow rigimes. Second, isothermal convective mass transfer in a channel is considered, assuming the diffusing substance to be an ideal gas with Lewis number equal to unity. The flow is considered to be either laminar or turbulent with boundary conditions at the channel walls of either constant concentration or constant mass flux. The analogy between heat and mass transfer is used to determine the entropy generation and the relations for optimum plate spacing and Reynolds number. The applicable range of the results for both limiting cases are then investigated by non-dimensionalizing the entropy generation equation.

Entropy generation in combined heat and mass transfer

Abstract Irreversible entropy generation for combined forced convection heat and mass transfer in a twodimensional channel is investigated. The heat and mass transfer rates are assumed to be constant on both channel walls. For the case of laminar flow, the entropy generation is obtained as a function of velocity, temperature, concentration gradients and the physical properties of the fluid. The analogy between heat and mass transfer is used to obtain the concentration profile for the diffusing species. The optimum plate spacing is determined, considering that either the mass flow rate or the channel length are fixed. For the turbulent flow regime, a control volume approach that uses heat and mass transfer correlations is developed to obtain the entropy generation and optimum plate spacing.

Second-law analysis of a wet crossflow heat exchanger

A second-law analysis of a wet crossflow heat exchanger is performed for various weather conditions. The heat exchanger can be used as an energy-saving device for ventilation in air-conditioning. The heat and mass transfer is solved by using the model developed by Holmberg. The effectiveness, exergy recovery factor and second-law efficiency of the wet heat exchanger are individually defined. The effects of lateral solid heat conduction on the effectiveness, exergy recovery factor and second-law efficiency are numerically investigated for various operating conditions. Two optimum design criteria, one for the maximum second-law efficiency and the other for the maximum exergy recovery factor, are obtained.

Heat and mass transfer in a two-dimensional cross-flow regenerator with a solid conduction effect

An analysis of a two-dimensional cross-flow heat and mass regenerator with conductive heat transfer is presented. The adsorbent is a regular density silica gel. Non-switched and switched operations are considered. An enthalpy effectiveness factor is used to specify the regenerator performance. Eight coupled non-linear partial differential equations specify the process of heat and mass transfer. The equations are solved using a finite difference method. The average Lewis number and Cmin/Cmax are assumed to be unity. The effects of Ntu, Cr/Cmin, desiccant content, Biot number and outdoor climate condition on the regenerator performance are analyzed. Both parallel-cross-flow and counter-cross-flow arrangements for sorption are considered. The performance of an ideal regenerator with infinite Ntu and an isothermal wall is analyzed.

Effect of axial solid heat conduction and mass diffusion in a rotary heat and mass regenerator

Abstract This research analyzes the effect of axial heat conduction and mass diffusion on the performance of a solid desiccant wheel. A one-dimensional transient heat and mass transfer model which contains four nonlinear partial differential equations is developed. The equations are solved using a full-implicit finite-difference thod. A limiting case with a zero solid diffusion resistance is considered. Three different regeneration temperatures, namely 26, 50 and 85 C, are considered. The parameter, ( λ 4 2 Ntu/Bi ), is found to be the most important factor governing the axial heat conduction and mass diffusion effect.

Modeling and testing of a silica gel packed-bed system

Abstract A two-column packed-bed desiccant dehumidification system is tested and analytically modeled. The results are consistent. The desiccant is a spherical silica gel particle with an average diameter of 5 mm. The length of the columns are individually 9, 15 and 24 cm. Three regeneration temperatures are used, namely, 65, 75 and 85°C. An optimum operating time period is obtained. The heat and mass transfer is analyzed by using a solid-side resistance method with consideration of a fluid friction effect. The magnitude of the fluid friction effect is governed by the Reynolds number and a nondimensional length factor. For a long column or a high inlet air velocity the fluid friction effect becomes significant.

Measurement of apparent solid-side mass diffusivity of a water vapor–silica gel system

Abstract A measurement of the apparent solid-side mass diffusivity of water vapor adsorbed in a regular density silica gel is performed by using a constant-pressure thermal gravimetrical apparatus. The diameter of the silica gel particles is 2 mm. Six adsorption isotherms, individually correspond to 5.1, 22.2, 34.3, 49.5, 64.4 and 79.6 °C, are measured. The covered range of moisture content is from 0% to 40%. Using a previously developed model, which considers both surface (film) heat and mass transfer resistances, the measured uptake curves yield the apparent solid-side mass diffusivities. The apparent solid-side mass diffusivity is expressed as a function of temperature and moisture content. The thermal effect and importance of surface mass transfer resistance are individually discussed.

Mass diffusion in a spherical microporous particle with thermal effect and gas-side mass transfer resistance

Abstract An analytical solution for mass diffusion in a spherical microporous particle experienced with a small step change of gaseous phase adsorbate concentration is obtained. The mass diffusion in the solid is assumed to be micropore diffusion controlled. Thermal effect and gas-side mass transfer resistance are considered. The governing equations are solved by using the Laplace transformation method. Three factors, α , β and γ are defined to govern the heat and mass transfer. α and β are relevant to the thermal effect and γ dominates the gas-side mass transfer resistance. The applicable ranges of three simplified models are discussed. A limiting solution with mass diffusivity approaching infinity is obtained.