A.S. Mujumdar

CONVECTIVE DRYING OF BANANA, GUAVA AND POTATO PIECES : EFFECT OF CYCLICAL VARIATIONS OF AIR TEMPERATURE ON DRYING KINETICS AND COLOR CHANGE

ABSTRACT Pieces of banana, guava and potato were dried in a two-stage heat pump dryer capable of precise control of air humidity with predetermined cyclic variations of air temperature entering the drying chamber. The air temperature variations tested were : a cosine, a reversed cosine and three different square wave profiles with peak-to-valley variations from 20°C to 40°C. The cycle time was about 60 minutes with drying time of approximately 300 minutes. The drying samples were placed on trays in a thin layer. With appropriate choice of temperature-time variation, it is possible to reduce the overall color change while maintaining high drying rates.

Comparative evaluation of different turbulence models for confined impinging and opposing jet flows

A numerical investigation was carried out to predict and compare the fluid flow and heal transfer characteristics of two-dimensional turbulent confined impinging and opposing jet flows. Five low-Reynolds-number k-∊ models and the standard high-Reynolds-number model were used in the simulation. The newly proposed Yap correction was also tested with low-Reynolds-number models in order to investigate its effect on the heat transfer predictions for the impinging jet case. It was found that in some models this correction improves the heat transfer predictions. A comparison is made between the simulation results and the available experimental data for fluid flow and heat transfer for a single slot jet impinging on a flat plate and for flow in the opposing jet case. A parametric study is made for flow and heat transfer characteristics in the opposing jet configuration.

A Mathematical Model for Constant and Intermittent Batch Drying of Grains in a Novel Rotating Jet Spouted bed

ABSTRACT A diffusion-bared mathdcal model is prswmed for batch drying of corn in a novel rotating jn spouted bcd device under constant as well as intermitted drying conditions. Such a device is suited for drying of large partides (e.g. grains, beam, seed, etc) for which internal heat and mass tranfer rates control the drying kinetics. Based on literature data for moisture diffusivities the model predictions are compared with experimental data for both continous and time-dependend air supply and/or heat input. Effects of dcvant parameter are evaluated and discussed in the light of potential pnrctieal applications.

INTERMITTENT DRYING OF CARROT IN A VIBRATED FLUID BED: EFFECT ON PRODUCT QUALITY

ABSTRACT Drying of diced carrot was studied experimentally in a laboratory batch-type vibrofluidized bed dryer (VFB) The kinetics of the degradation reaction of P-carotene chosen as the dominant quality index were determined for various time-temperature conditions. Relationships were obtained for instantaneous concentration of β-carotene, and for the degradation rate constant as a function of moisture content and material temperature. It was shown that the best quality product is obtained when carrot is dried in a VFB dryer down to the critical moisture content, than conditioned (tempered) in a single layer exposed to ambient air, and then finish dried to the final moisture content in a VFB dryer.

Numerical Prediction of the Flow Field and Impingement Heat Transfer Caused by a Laminar Slot Jet

The effects of uniform suction and nozzle exit velocity profile on the flow and heat-transfer characteristics of a semiconfined laminar impinging slot jet were investigated numerically. The full Navier-Stokes and energy equations were solved using a hybrid or upwind finite-difference representation of the equations cast into their vorticity-stream-function form. The importance of the nozzle exit profile is shown by comparison of the computed heat-transfer distribution with the available experimental data in the laminar range. Application of suction at the impingement surface is shown to enhance the local heat-transfer rates by a constant amount. The nondimensional heat-transfer coefficient and skin friction at the plate are computed as functions of the nozzle Reynolds number, the suction rate, and the nozzle velocity profile. The effect of temperature-dependent physical properties is included in the analysis.

Flow and thermal characteristics of steady two dimensional confined laminar opposing jets: Part II. Unequal jets

Results of numerical simulation of the flow, mixing and thermal characteristics of a steady laminar two-dimensional confined opposing jet configuration are provided for the case when the two jets are unequal. Effects of geometric, hydrodynamic and thermal parameters on the flow and heat transfer characteristics of such systems are examined.

DRYING OF PASTES IN SPOUTED BEDS OF INERT PARTICLES : DESIGN CRITERIA AND MODELING

ABSTRACT The drying of pastelike materials can be performed well in spouted beds (SB) of inert particles. In this work the drying performance of pastes in conical pastes in conical spouted beds is analyzed as a function of column dimensions, fluid flow characteristics and paste properties. imulated data on fluid flow together with the experimental results on drying of different pastelike materials are presented and discussed to provide criteria for the design of a conical spouted bed dryer for suspensions.

Microwave Drymg of Grapes in a Single Mode Cavity at 2450 Mhz - i: Drying Kinetics

ABSTRACT Grapes were dried into raisins using combined microwave and convective drying in a single mode cavity specially designed for the studies. Effect of processing parameters, viz.microwave power density (P) air temperature (T) and velocity (u) on drying kinetics was studied in an effort to optimize the operating conditions. The individual and interactive effects of these variables were srudied using three levels of P, four levels of T and two levels of u. Higher drying rates observed with the higher P and T were atuibuted to the increased product temperature and higher partial vapour pressure difference between the surface of the fruit and the air stream. Increase of air velocity increased the drying times. Low drying rate at higher air velocity was attributed to decreased fruit temperature due to rapid convective heat mnsfer at the surface.

Fundamental particle mixing studies in an agitated bed of granular materials in a cylindrical vessel

Abstract Experimental results are presented on the fundamental particle mixing phenomena in a bed of granular material agitated by a moving blade in a two-dimensional vessel. The study is the first step aimed at evaluating the dryer wall-to-particle heat transfer coefficient for drying operations from the knowledge of fundamental particle mixing. The parametric effects of the height of the blade, bed depth and blade velocity for a fixed clearance between the wall and the blade are discussed for a bed of glass beads. It was found that the ratio of the bed to blade height might be more important than the blade speed in influencing movements within the bed.

DEFORMATION AND SlRESS ANALYSIS OF POROUS CAPILLARY BODIES DURING INTERMITTENT VOLUMERIC THERMAL DRYING

ABSTRACT lntermittent heating in a thermal drying is one way to improve energy utilization and to enhance the quality for some products. Using the finite element method and Luikovs theory. the temperature, moisture and stress distributions at different instances of intermittent drying were obtained. Several examples were analyzed using this method. The numerical results are compared with prior experimental and theoretical results. This numerical simulation method could be used to devise an intermittent drying process schedule optimal operation.