Showing papers by "Arun S. Mujumdar published in 2002"

Sludge dewatering and drying

Abstract: The production of wastewater treatment sludge, the basic characteristics of the sludge and the state of the water in the sludge are described in this paper. The methods for the determination of bound water content are discussed. The literature (including patents) on sludge dewatering and drying is reviewed, including vacuum filters, belt presses, centrifuges, direct dryers, indirect dryers and combined mode drying systems. The issues related to drying are briefly discussed. Photographs or schematics of typical dewatering and drying systems are also included.

Flow and heat transfer for gas flowing in microchannels: a review

Abstract: Microchannels are currently being used in many areas and have high potential for applications in many other areas, which are considered realistic by experts. The application areas include medicine, biotechnology, avionics, consumer electronics, telecommunications, metrology, computer technology, office equipment and home appliances, safety technology, process engineering, robotics, automotive engineering and environmental protection. A number of these applications are introduced in this paper, followed by a critical review of the works on the flow and heat transfer for gas flowing in microchannels. The results show that the flow and heat transfer characteristics of a gas flowing in microchannels can not be adequately predicted by the theories and correlations developed for conventional sized channels. The results of theoretical and experimental works are discussed and summarized along with suggestions for future research directions.

Simulation of convection-microwave drying for a shrinking material

Abstract: The transient conservation equations for heat and mass transfer in a drying solid were solved using a finite element technique. The material being dried is a discretely non-homogeneous material. It is subjected to a convection boundary condition. The shrinkage of the material surrounding a non-shrinking, low loss factor material is treated numerically by continuously redefining the computational domain of the problem. Results of the numerical prediction of drying rates and temperature distribution within the drying solid were compared with experimental results and found to be in good agreement. Local convection heat transfer boundary conditions were prescribed with the use of a computational fluid dynamic code. Cylindrical pieces of carrot with a coaxial cylindrical insert of Teflon were used as the non-homogeneous models for the experimental and computational study.

Computational Study of Impingement Heat Transfer under a Turbulent Slot Jet

Abstract: Impinging jets of various configurations are commonly used in numerous industrial applications such as drying of tissue, paper, textiles, and photographic films and cooling of high power density electronic components, because of their highly favorable heat and mass transfer characteristics. Despite an extensive body of research literature on the subject over the past two decades, impinging jet heat transfer remains an active area of research covering both the experimental and computational fluid dynamics aspects because of its inherently complex fluid and heat flow characteristics. This paper presents simulation results for a single semiconfined turbulent slot jet impinging normally on a flat plate. Effects of turbulence models, near wall functions, jet turbulence, jet Reynolds number, as well as the type of thermal boundary condition at the target surface are discussed in the light of experimental data. The computed results compare favorably with the experimental results for large nozzle-to-target spacin...

Optimum operating conditions in drying foodstuffs with superheated steam

Abstract: It is inferred from experimental data that in drying foodstuffs with superheated steam, the initial drying rate has a direct effect on the rate at which the overall drying takes place. That is, the faster the initial drying rate, the shorter the overall drying time. This criterion is very convenient because at the beginning, water moistens the sample external surface so evaporation does not depend on internal sample characteristics, but only on external convective heat and mass transfer rates. Mass and energy balance equations are solved and the result converted into a general initial drying rate equation, in which all dryer characteristics are grouped into one dimensionless parameter. The initial drying rate equation is mathematically maximized and the optimum working conditions determined. The result shows that initial drying rate always increase with increases of either the superheated steam temperature or velocity, but once these two variables are fixed, there exists at least one “optimum” pr...

Simulation of fluidized-bed drying of carrot with microwave heating

Abstract: A mathematical model of coupled heat and mass transfer was applied to batch fluidized-bed drying with microwave heating of a heat sensitive material—carrot. Four kinds of microwave heating with intermittent variation were examined. The numerical results show that different microwave heating patterns can affect the fluidized bed drying significantly. Changing the microwave input pattern from uniform to intermittent mode can prevent material from overheating under the same power density. Supplying more microwave energy at the beginning of drying can increase the utilization of microwave energy while keeping temperature low within the particle. For a particle diameter of 4 mm, fluidization velocity of 2 m/s, inlet airflow temperature of 70°C and the bed area factor of 80, the drying time are 750 and 1000 s, respectively, for the two good operating conditions with on/off periods of 125/375 s and 375/375 s. The cumulative microwave energy absorbed by particles at the end of drying is 1415 and 2300 kJ/...

Effect of large temperature differences on local nusselt number under turbulent slot impingement jet

Abstract: In heating, cooling or drying applications involving large temperature differences between the jet and the target surface, it is necessary to incorporate the temperature-dependence of fluid properties on the flow and temperature fields. Despite their frequent occurrence in industrial practice, there is little research reported in the literature on this subject. It is also necessary to distinguish between heating and cooling applications since the thermo-physical properties of the fluid in the vicinity of the target surface vary in different directions for the two cases. The objective of this work is to present computational fluid dynamic model results for heat transfer under a semi-confined slot turbulent jet under thermal boundary conditions such that the temperature-dependence of the fluid properties affects the flow and thermal fields. A comparative analysis in the turbulent flow regimes is made of the standard k–e and Reynolds stress turbulence models for constant target surface temperature. ...

Heat transfer characteristics in a pulsating fluidized bed in relation to bubble characteristics

Abstract: A pulsating fluidized bed is operated with two sequential durations designated as an on-period with injecting fluidization gas and an off-period without it. The heat transfer coefficient between a vertically immersed heater and bed in a pulsating fluidized bed is measured under various pulse cycles and fluidized particles. The obtained results are compared with those in a normal fluidized bed with continuous fluidization air injection. The relationship between heat transfer coefficients and bubble characteristics, evaluated using a digital video camera, has also been investigated. For certain fluidized particles and operating pulse cycles, the fluidization of particles and the increment of heat transfer coefficients can be obtained under a mean air velocity based on a pulse cycle duration smaller than the minimum fluidization air velocity in a normal fluidized bed. Under the pulse cycles where a static bed through the whole bed is formed in the off-period duration, the improved heat transfer rate over that in a normal fluidized bed can be measured. This may be attributed to large bubble formation. As heat transfer in the pulsating fluidized bed is obstructed with increasing time to keep a static bed due to the excessive off-period duration, it is indicated that there is an optimum off-period duration based on the heat transfer rate. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 307–319, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10038

Theoretical Study of Microwave Heating Patterns on Batch Fluidized Bed Drying of Porous Material

Abstract: Abstract The effects of microwave heating patterns were investigated for the case of batch fluidized bed drying by solving numerically a coupled heat and mass transfer model. Three patterns, uniform, sinusoidal and rectangular waves, were examined with the average electric field strength kept constant at 1000 V / m . Properties of apple were used to represent porous materials. The results show that the magnitude and distribution of the moisture, temperature and pressure within a particle can be significantly affected. More importantly, the drying time and microwave energy consumption change with the pattern of how microwave heating is applied. Particularly, intermittent heating with a rectangular wave pattern has the most microwave energy consumption but the shortest drying time. For a particle size of 5 mm , airflow rate of 2 m / s and temperature of 60°C, the bed area factor fA of 100, the drying time is, respectively, 1600, 2000, 2400 s for rectangular, sinusoidal, and constant microwave heating. The values of microwave energy consumed for the three cases are, respectively, 2145, 1980, and 1560 kJ / kg water evaporated.