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Convective Heat Transfer Coefficient for Indoor Forced Convection Drying of Corn Kernels

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Convective Heat Transfer Coefficient for Indoor Forced Convection Drying of Corn Kernels Int. J. Mech. Eng. & Rob. Res. 2013 Ravinder Kumar Sahdev et al., 2013 ISSN 2278 – 0149 www.ijmerr.com Vol. 2, No. 4, October 2013 © 2013 IJMERR. All Rights Reserved Research Paper CONVECTIVE HEAT TRANSFER COEFFICIENT FOR INDOOR FORCED CONVECTION DRYING OF CORN KERNELS Ravinder Kumar Sahdev1*, Chinu Rathi Saroha1 and Mahesh Kumar2 *Corresponding Author: Ravinder Kumar Sahdev, [email protected] In this present research paper, an attempt has been made to determine the convective heat transfer coefficient of corn kernels under indoor forced convection drying mode. The experiments were conducted in the month of May 2013 in the climatic conditions of Rohtak (28° 40': 29 05'N 76° 13': 76° 51'E). Corn kernels were dried from initial moisture content 43% dry-basis. Experimental data was used to evaluate the values of constants (C and n) in Nusselt number expression by using linear regression analysis and consequently convective heat transfer coefficient was determined. The convective heat transfer coefficient for corn kernels was found to be 1.04 W/m2 °C. The experimental error in terms of percent uncertainty has also been calculated. Keywords: Corn kernels, Convective heat transfer coefficient, Indoor forced convection drying INTRODUCTION the interior of the corn kernels takes place due Corn is one of the main agricultural products to induced vapor pressure difference between in many countries. It is an important industrial the corn kernels and surrounding medium. raw material in the starch industry (Haros and The convective heat transfer coefficient is Surez, 1997; and Soponronnarit et al., 1997a). an important parameter in drying rate It is also a grain that can be eaten raw off the simulation since the temperature difference cob. Traditionaly corn kernels are dried in open between the air and corn kernels varies with atmosphere exposing them to the sun. Open this coefficient. Sodha et al. (1985) presented sun drying is the most primitive methods of corn a simple analytical model based on kernels drying. The removal of moisture from simultaneous heat and mass transfer at the 1 Mechanical Engineering Department, University Institute of Engineering & Technology, Maharshi Dayanand University, Rohtak, India. 2 Mechanical Engineering Department, Guru Jambheshwar University of Sciences & Technology, Hisar, India. 18 Int. J. Mech. Eng. & Rob. Res. 2013 Ravinder Kumar Sahdev et al., 2013 product surface and included the effect of wind convection mode. The values of convective heat speed, relative humidity, product thickness, transfer coefficient for shredded, slice and and heat conducted to the ground for open sun pieces forms were found to be 30.39, 25.88, drying and for a cabinet dryer. Goyal and Tiwari and 18.67 W/m2 °C respectively. Ravinder (1998) have studied heat and mass transfer et al. (2012) studied the drying of vermicelli in in product drying systems and have reported indoor forced convection drying mode. The the values of convective heat transfer convective heat transfer coefficient was found coefficient for wheat and gram as 12.68 and to vary from 0.98-1.10 W/m2 °C. 9.62 W/m2 °C, respectively, by using the simple In the present research paper, the convective regression and 9.67 and 10.85 W/m2 °C heat transfer coefficient has been found for corn respectively, for same products while using the kernels under indoor forced convection drying multiple regression technique. Anwar and mode. The value would be helpful in designing Tiwari (2001a) studied the drying of six crops a dryer to dry corn kernels to its optimum (green chilies, green peas, white gram, onion storage moisture level of about 16%. flakes, potato slices and cauliflower) under forced convection drying mode. The values of MATERIALS AND METHODS convective heat transfer coefficients were found Experimental Set-Up and to vary between 1.31 and 12.80 W/m2 °C and Procedure between 1.25 and 10.94 W/m2 °C in indoor open and closed conditions under forced mode A rectangular shaped wire mesh tray of 2 respectively. Akpinar (2004) determined the dimension 196×155 mm was used to convective heat transfer coefficient of various accommodate the corn kernels. A digital agricultural products, namely, mulberry, weighing balance (Smart, Aqua Series) of 6 strawberry, apple, garlic, potato, pumpkin, kg capacity having a least count of 0.1 g was eggplant, and onion under indoor forced convection used to measure the mass of moisture drying mode. The convective heat transfer evaporated. A non-contact (infra-red coefficient of these crops was found to vary from thermometer) thermometer (Raytek-MT4) crop to crop between 0.644-7.121 W/m2 °C. Togrul having a least count of 0.2 °C with an accuracy (2003 and 2005) have determined the of ± 2% on a full scale range of –1 to 400 °C convective heat transfer coefficients of some was used to measure the surface temperature crops dried under open sun conditions. Kumar of corn kernels. An eight channel digital et al. (2011) studied the drying of papad in open temperature indicator (0-200 °C, least count sun and indoor forced convection drying of 0.1 °C) with a calibrated thermocouple was modes. The convective heat transfer used to measure the ambient temperature. A coefficients of papad were found to be 3.54 and digital hygrometer (model Lutron HT-315) was 1.56 W/m2 °C under open sun drying and indoor used to measure the relative humidity and forced convection drying modes respectively. temperature of exit air. A heat convector (Usha Anwar et al. (2012) studied the drying of Indian Shriram, model FH-812T, 230V) was used to goosberry (Embica officinalis) in three forms blow hot air over the corn kernels surface (shrede, slices and pieces) under forced during the forced convection mode. 19 Int. J. Mech. Eng. & Rob. Res. 2013 Ravinder Kumar Sahdev et al., 2013 Experiments were conducted in the month Sample Preparation of May 2013 for indoor forced convection Corn cobs were purchased from the local drying mode in the climatic conditions of market and grains (corn kernels) were Rohtak (28° 40’: 29 05’N 76° 13’: 76° 51’E). separated from it. The corn kernels of 82.9 The corn kernels were kept on the weighing grams were used for indoor forced convection balance using the wire mesh tray. A digital drying mode. hygrometer was kept after corn kernels surface with its probe facing the exit air to measure Thermal Modeling the humidity and temperature of the exit air. The convective heat transfer coefficient for Every time it was kept on 1 minute before indoor forced convection drying mode has reading the observations. The air velocity of been calculated by using the expression for heat convector was measured with the help of Nusselt number as (Tiwari and Suneja, 1977; digital anemometer (model Lutron AM-4201) and Kumar et al., 2011): and it was observed to be 1.5 m/s. All the h X n observations were recorded at every 10 Nu c CRePr minute time interval. The difference in weight Kv directly gave the quantity of water evaporated or during that time interval. Average values of corn n kernels surface temperature Tc , exit air Kv hc CRePr ...(1) temperature Te and relative humidity were X calculated from the two consecutive values for The rate of heat utilized to evaporate that time interval and were used in the moisture is given as (Malik et al., 1982) calculations. The photograph of the experimental set up under indoor forced Qe 0.016hc PTc P Te ...(2) convection drying mode is shown in Figure 1. On substituting h from Equation (1), The experiment was repeated twice for c Equation (2) becomes obtaining more accurate results. Kv n Figure 1: A Photograph of Experimental Qe 0.016 CRePr PTc PTe Set-Up for Indoor Forced Convection X Drying Mode ...(3) The moisture evaporated is determined by dividing Equation (3) by the latent heat of vaporization () and multiplying by the area of the tray (At) and time interval (t) Q m e t A ev t K n 0.016 v CRePr PT PT t A X c e t ...(4) 20 Int. J. Mech. Eng. & Rob. Res. 2013 Ravinder Kumar Sahdev et al., 2013 K 0.016 v PT PT t A Z 5144 Let c e t PT exp25.317 ...(11) X T 273.15 mev n where CRePr ...(5) Z T T c e Taking logarithm on both sides of equation Ti 2 mev The values of constants C and n have been ln lnC nlnRePr ...(6) Z determined by linear regression analysis by using measured data of the corn kernels and This is the form of a linear equation, exit air temperature, exit air relative humidity Y mX 0 C0 and moisture evaporated during a certain time where period. The following linear regression formulae have been used to calculate C and n mev Y ln , m n , N X Y X Y Z o 0 0 n 2 2 ...(12) No X 0 X 0 X lnRePr, C lnC 0 0 and Thus, C eC0 X 2 Y X X Y 0 0 0 The experimental constants (C and n) were C0 2 2 ...(13) No X0 X0 obtained from the above equations which were further used to determine the convective heat The experimental error was also calculated transfer coefficient. in terms of % uncertainty (internal + external). The following equations were used to evaluate The physical properties of humid air, i.e., % uncertainty (Nakra and Choudhary, 1991). specific heat Cv , thermal conductivity Kv , 2 2 2 2 density v , viscosity v and partial vapor .... U 1 2 3 n ...(14) pressure were calculated using the following N expressions (Anwar and Tiwari, 2001b; and where is the standard deviation and is given Kumar et al., 2011): as: 4 2 Cv 999.2 0.1434Ti 1.10110 Ti 2 X i X i 8 3 ...(15) 6.758110 Ti ...(7) N0 4 Kv 0.0244 0.7673 10 Ti ...(8) where Xi is the moisture evaporated and is the deviation of the observations 353.44 (Xi Xi ) v ...(9) from the mean.
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