Leon R. Glicksman

An investigation of heat transfer and friction for rib-roughened surfaces

Abstract An investigation of rib-roughened surface was undertaken to determine the effects of rib shape, angle of attack and pitch to height ratio on friction factor and heat-transfer results. A parallel plate geometry was used. Based on the law of the wall similarity and the application of the heat-momentum analogy developed by Dipprey and Sabersky, a general correlation for friction factor and heat transfer was developed to account for rib shape, spacing and angle of attack. Ribs at a 45° angle of attack were found to have superior heat transfer performance at a given friction power when compared to ribs at a 90° angle of attack or when compared to sand-grain roughness.

Scaling relationships for fluidized beds

Abstract The independent nondimensional parameters governing the dynamics of fluidized beds are found by nondimensionalizing the differential equations governin By proper adjustment of the length scales, the particle density, and the bed superficial velocity it is possible to obtain exact similitude between geo

Design analysis of single-sided natural ventilation

Natural ventilation is an effective measure to save energy consumed in buildings and to improve indoor air quality. This investigation studied single-sided natural ventilation by using a computational fluid dynamics (CFD) model, together with analytical and empirical models. The CFD model was applied to determine the effects of buoyancy, wind, or their combination on ventilation rates and indoor conditions. For buoyancy-driven flow, the CFD results are within a 10% difference from the semi-analytical results. For combined wind- and buoyancy-driven flow, CFD may have underpredicted the empirical model results by approximately 25%. This investigation also studied the effects of opposing buoyancy and wind forces.

Simplified scaling relationships for fluidized beds

Abstract Simplifications to the full set of scaling parameters for dynamic similarity of fluidized beds, Glicksman, Chemical Engineering Science, 39 (1984) 373, were explored. A new set of simplified scaling laws includes the Froude number based on column height, the solid to gas density ratio, the ratio of superficial to minimum fluidization velocity, bed geometric ratios, and particle sphericity and size distribution. When the gas to particle drag is represented by either the Ergun equation or a single particle drag equation, the new simplified laws hold exactly in both the viscous dominated and gas inertia dominated limits. For intermediate conditions, the gas to particle drag is well approximated in models based on the simplified scaling laws. The simplified scaling laws allow very small models to be constructed which properly simulate the hydrodynamics of a full size reactor or combustor. Experimental confirmation of the new simplified scaling laws and the viscous limit scaling parameters, where equality of the density ratio is omitted, were carried out in circulating fluidized beds. Within the viscous limit, the solid to gas density ratio is an important modeling parameter when the slugging regime is approached. In general, the solid to gas density must be matched to achieve good similarity. Using the new simplified scaling laws, good agreement was observed even when the length scale of the air fluidized model was as small as 1/16 that of an atmospheric combustor.

Dropwise condensation—The distribution of drop sizes

Abstract The nature of the drop growth process during dropwise condensation, as revealed by high magnification cine films [16–18, 29], is used as the basis of a simplified model of the sequence of events occurring during the growth cycle (i.e. The time interval between successive sweepings of the region of the surface under consideration). The model is used to predict the average distribution of drop sizes. The theoretical distribution is compared with measurements [8, 31], a recent computer simulation [32] and an earlier empirical distribution [28].

Radiation heat transfer in foam insulation

Abstract The contribution of radiation heat transfer to the overall conductivity of foam insulations has been examined. The absorption and scattering coefficients as well as the phase function were measured for foam and glassfiber insulation. A simple absorption coefficient can be derived from transmissivity measurements over a range of sample thickness. When this absorption coefficient is used in the diffusion equation to predict the radiative flux for foams, the error in the calculated radiant flux is of the order of ten percent for foams. For fibrous insulation, transmissivity measurements with an integrating sphere are necessary.

Numerical simulation of dropwise condensation

Abstract A numerical simulation of dropwise condensation heat transfer was performed. The simulation accounts for growth, coalescence, and renucleation of drops ranging in size from the smallest nucleating drops to departing drops. The calculations were performed with an active site density as high as 109 sites/cm2, Heat transfer to the drop was determined by considering the effects of curvature of the drop surface, interfacial mass transfer between the liquid and vapor phases, and conduction through the drop. The effects of non-condensibles in the vapor and non-uniform conduction in the condensing surface material were omitted. The heat transfer coefficient was found as a function of the active site density, the saturation temperature, the departing drop size, and the vapor to surface temperature difference. The results agree well with available experimental results.

Simulation of wind driven ventilative cooling systems for an apartment building in Beijing and Shanghai

Abstract This paper presents a performance evaluation of two passive cooling strategies, daytime ventilation and night cooling, for a generic, six-story suburban apartment building in Beijing and Shanghai. The investigation uses a coupled, transient simulation approach to model heat transfer and airflow in the apartments. Wind-driven ventilation is simulated using computational fluid dynamics (CFD). Occupant thermal comfort is accessed using Fanger’s comfort model. The results show that night cooling is superior to daytime ventilation. Night cooling may replace air-conditioning systems for a significant part of the cooling season in Beijing, but with a high condensation risk. For Shanghai, neither of the two passive cooling strategies can be considered successful.

Experimental verification of scaling relationships for fluidized bed

Abstract To verify the scaling relationships for fluidized beds, an atmospheric fluidized bed combustor and a cold bed constructed to operate at the same values

CIRCULATING FLUIDIZED BED HEAT TRANSFER

Abstract Heat transfer to the walls of a circulating bed is due to conduction from clusters of particles falling along the walls, thermal radiation, and convection to uncovered surface areas. Important hydrodynamic factors are the fraction of the wall covered by particles and the average contact time of particles at the wall. Small active heat transfer probes will give an upper limit to the heat transfer in a circulating bed. Experimental results show lower heat transfer coefficients for larger walls; heat transfer is limited by the temperature change of particle clusters moving down the walls. The contact time for clusters is related to their average displacement before breakup. For displacements of 15 cm or less, the cluster displacement approximates constant gravitational acceleration. The most pressing needs for commercial bed designers are heat transfer and hydrodynamic results valid for large diameter beds.