Resistance to Airflow of Paddy (Rough Rice) in Shallow Depths

Transcription

1 Resistance to Airflow of Paddy (Rough Rice) in Shallow Depths S. Gunasekaran, V. K. Jindal, G. C. Shove MEMBER ASAE MEMBER ASAE ABSTRACT AN experimental research was conducted to obtain information on resistance of paddy to very low airflows. To simulate natural convection, airflows in the wide range of to 0.3 m 3 /s-m 2 were created by establishing temperature gradients and by means of a suction fan. Pressure drops across 5, 10, 15, 20, 25 and 30 cm deep grain beds were measured using suitably developed instruments. The effects of grain bed depths and airflow rates on resistance pressure were analyzed. The increase in pressure drop across the grain bed was proportionate with bed depth, but was disproportionate with an increase in airflow rate. These results, in general, are in good agreement with those of other researchers. An approximate equation was also developed for possible application in the design of solar grain dryers employing natural convective airflow. i -f- > A REFERENCE /TRANSISTOR SENSOR TRANSISTOR PERSPEX TUBE fh SENSOR TRANSISTOR INTRODUCTION It has long been observed that the diurnal and seasonal variations in the ambient conditions induce temperature gradients within large grain masses. Such gradients cause convective intergranular air currents which carry moisture from one place to another (Muir, 1973; Stewart, 1975) creating localized high moisture regions. This results in increased microbial and insect activity leading to grain quality deterioration (Hukill, 1953; Converse et al., 1973). In the past there has been very little effort (Fenton, 1941; Johnson, 1957) to quantify the low movements of air within the grain mass due to temperature gradients. Estimation of such airflows would provide basic information regarding the moisture movement wihin the grain mass. Moreover, the need for aerating grains to prevent moisture migration has made it necessary to obtain the information on resistance of grains to very low airflow rates. This article presents simple instrumentation and measurement techniques for low airflow and pressure drop measurements. The results of experiments on resistance of paddy grain beds to airflows in the wide range of to 3 m 3 /s*m 2, created both by mechanical means and induced temperature gradients are also presented. INSTRUMENTATION Instruments for very low air velocity and pressure drop measurements were not readily available. Hence, Article was submitted for publication in June, 1982; reviewed and approved for publication by the Electric Power and Processing Div. of ASAE in October, Presented as ASAE Paper No The authors are: S. GUNASEKARAN, Research Assistant, Agricultural Engineering Dept., University of Illinois, Urbana; V. K. JINDAL, Associate Professor, Agricultural and Food Engineering Division, AIT, Bangkok, Thailand; and G. C. SHOVE, Professor, Agricultural Engineering Dept. University of Illinois, Urbana. OUTLET CROSS SECTION AT "A A" 1» Zl T Fig. 1 Apparatus used for low airflow calibration. suitable instruments were developed and calibrated in the anticipated range of air velocities and pressure drops. For low air velocity measurements a transistor anemometer based on the one suggested by McHattie (1979) was developed. It makes use of the property that a transistor at constant current loses about 2 mv/c almost linearly with temperature. When subjected to airflow the cooling effect causes a change in transistor temperature. The corresponding base-to-emitter voltage was directly related to air velocity. The ambient temperature was obtained from a reference transistor connected as a diode and operated at negligible dissipation. A special probe was designed to mount the sensor transistor and allow it to be fully exposed to airflow. In the air velocity range of 0.01 to 0.03 m/s calibration was done using the airflow created by raising the level of water in a tube at a constant rate. The apparatus used for this is shown in Fig. 1. To ensure a constant rate of airflow, water was let into the tube at the top from a constant head tank. The bottom opening was used only to empty the tube. In the higher air velocity range of 0.2 to 0.3 m/s a blower was used for calibration in conjunction with a vane anemometer. Fig. 2A and 2B are the calibration curves obtained. The instrument showed an instability of about ± 5 to 15% in the air velocity calibration of 0.01 to 0.03 m/s. However, it was quite reliable in 0.2 to 3 m/s range with 1983 TRANSACTIONS of the ASAE 1983 American Society of Agricultural Engineers /83/ $

2 CANTILEVER \ SMALL ~ T ~ CONTAINER y \ SEMI CONDUCTOR STRAIN GAGES L AIR VELOCITY, cm/s Fig. 2A Calibration of transistor anemometer for low airflow. 45 cm FLEXIBLE TUBE^ (4-23cm-- H Fig. 3 Apparatus used for low pressure measurement. AIR VELOCITY, Fig. 2B Calibration of anemometer for high airflow. transistor an uncertainty of less than 3%. In actual experiments, measurements were done mostly in the latter range due to magnification of air velocity at the sensor. The device for low pressure measurement was developed based on the principles of a well manometer. It consisted of two containers, one large and the other small, both connected by a flexible tube at the bottom (Fig. 3). The small one was hung at the free end of a thin high carbon steel cantilever. At the fixed end of the cantilever 4 semiconductor strain gages were mounted forming a full bridge configuration. The liquid level change in the large container due to applied pressure transferred a corresponding amount of liquid to the small one. This was mechanically amplified by the cantilever and the bridge output was taken as the measure of the pressure applied. The calibration was done with an inclined manometer sensitive to 9.81 Pa (1 mm water). However, since the gage output was very linear, it was extrapolated down (Fig. 4) to measure pressures as low as Pa (0.018 mm) water, corresponding to the lowest measurable digital volt meter (DVM) reading. APPARATUS The apparatus used for the airflow vs. pressure drop measurements was "U" shaped (Fig. 5). Total height of the apparatus was about 2.5 m and the inner cross " 3000 y DVM OUTPUT = (P) ^ / (R 2 = ) 2000 / (AMPLIFICATION FACTOR = 100 DVM SENSITIVITY = ImV) P <L i EXTRAPOLATION OF DVM OUTPUT FOR LOW PRESSURE MEASUREMENT PRESSURE, P, mm H20 Fig. 4 Calibration of low pressure measurement device. Fig. 5 Apparatus used for resistance of paddy to airflow measurement. 602 TRANSACTIONS of the ASAE 1983

3 TABLE 1. AIRFLOW INDUCED BY TEMPERATURE GRADIENTS THROUGH PADDY GRAIN BEDS. Temperature gradient, C Grain bed thickness, era Airflow, m /s-m section was 30 x 30 cm square. One of the vertical sections was of plywood and the remaining part was of perspex (clear plastic) sheet. The plywood section was well insulated with asbestos sheets from inside and equipped with a heater unit and temperature regulator at about 50 cm from the bottom. In the other vertical section a fine wire mesh was provided at the top to hold grain beds up to 30 cm deep. A pressure tap was provided immediately below the wire mesh. The tapping was a 6 mm diameter glass tube inserted well inside to avoid wall effects on pressure drop measurements. The horizontal portion was used for air velocity measurments. It was equipped with a replaceable velocity accelerator section to increase the air velocity by about 100 times at the transistor sensor. The probe was placed at a distance sufficiently away from the heater unit to keep the temperature effects on the transistor to a minimum. PROCEDURE Paddy or rough rice used for the experiments was a locally grown long grain variety in Thailand (RD-7). The average moisture content of the lot was 12.8% wet basis. The average bulk density was 548 kg/m 3 based on the measurements of actual weight and volume in test chamber. The size analysis using U.S. Standard Sieves No. 8 and No. 10 with openings of and mm, respectively, revealed that about 30% of paddy by weight was retained on No. 8 sieve and 70% on No. 10 sieve. Grain layers of 5, 10, 15, 20, 25 and 30 cm were formed by freely pouring the grain from a distance of about 50 cm and levelling the top by hand. Initially, to simulate airflow due to induced temperature gradients a suction fan was used to draw the air down through the grain bed. The airflow rates were controlled by adjusting a damper close to the fan end. Large air velocities were measured with the help of a sensitive vane anemometer and the low velocities with the transistor anemometer. Corresponding to each airflow rate, pressure drop across the grain beds was recorded. Later, the heater unit with temperature regulator was used to create temperature gradients of 10, 20, 30 and 40 C and create natural convective airflow and similar measurements were made. The airflow measurements with induced temperature gradients are given in Table 1. All experiments were replicated thrice, emptying and refilling the grains each time, to exclude the effects of filling. In the experiments with induced temperature gradients the whole apparatus was allowed to cool to room conditions before each trial. RESULTS AND DISCUSSION The resistance to airflow characteristics of grains are usually reported for a unit depth of grain bed. So, the experimental data at different bed depths were expressed on the basis of 1 m depth of grain. The results are presented in Figs. 6 and 7. It was observed that the data obtained with natural convective airflow established a usual liner relationship on logrithmic scale as, log(q) = log(a) + b log(ap) [1] Q = airflow rate, m 3 /s-m 2 AP = pressure drop, Pa/m a, b = product constants The values of 'a' and 'b' obtained by regression analysis are and 0.87 respectively. SPSS (Statisticsl Packages for the Social Sciences) routines were used for this and the following equation fitting purposes. The data obtained with the airflow created by a fan were found to conform better to a nonlinear model (R ), log (Q) = (log AP) log(ap) [2] rather than to a single linear model (R 2 = ), log (Q) =.81 log(ap) [3] PRESSURE DROP, P, Pa Fig. 6 Resistance of paddy to airflow created by a suction fan. PRESSURE DROP, P, Pa Fig. 7 Resistance of paddy to airflow due to natural convection TRANSACTIONS of the ASAE 603

4 Approximate airflow range, m 3 /s*m 2 Grain bed thickness, cm (computed) TABLE 2. PRODUCT CONSTANT VALUES FOR PADDY IN DIFFERENT AIRFLOW RANGES AIRFLOW CREATED BY SUCTION FAN. a i *>i a b a b Therefore, the data points were arbitrarily approximated by three linear segments in the airflow ranges of 0 to 0.002, to and to m 3 /s m 2. They each had a better coefficient of determination (R 2 ) value than that of equation [3] (Fig. 6). The equation so obtained is of the following form: log(q)= [log a! +b x log APJ + [log a 2 +.b 2 log AP 2 ] + [loga 3 +b 3 logap 3 ] [4] a u bi... etc. are constants The values of these constants are given in Table 2. Anyone of the three segments in equation [4] is to be used for calculation in corresponding airflow range and at transition points either the proceeding or the following segment can be used without any significant error. It can be easily observed that the resistance to airflow increased more rapidly with increase in air velocity which conforms with the observation of Shedd (1951), Sheldon et al. (1960), Husain and Ojha (1969) and Agrawal and Chand (1974). However, based on the present study it is difficult to substantiate Sheldon et al. (1960) observation of disproportionate increase in pressure drop with increase in grain bed depth. This may be due to the small bed depths used in our experiments. Within the beds the changes in density of fill were hardly appreciable to influence pressure drop values. Similar findings were reported by Husain and Ojha (1969) in paddy depths of 0.3 to 0.7 m. Fig. 8 presents a direct comparison of the trends of the results of this study with those of other researchers. Shedd (1951) reported a higher resistance for the same airflow rate. It may be due to increase in bulk density in this case as he used 2.47 m (9 ft) depth of paddy. The results of Husain and Ojha (1969), Agrawal and Chand ttttt= J_LJJ iiii JJJL iff l Jill 1 "IT T lull INK ##= lii] i ttii ' 1 L -L J 1 Tm ttr i HU AIN 8 1 "fffo $mf ~i -6 *6L 7 * 1 (WHEAT) PRESSURE DROP PER UNIT DEPTH, P, Po/m nil f" EEEE -Hi 1 fl U jjlj- W ' IT A AY / * s "j* Aw >." 9ei WAu- '111 nrr IP -V Y\ inn / / w \\\A \/A WI TH ffl ill Fig. 8 Comparison of different resistance of paddy to airflow data. fe (1974) and Wieneke (1977) are more comparable to the present one. In general at air velocities of 0.02 m/s and greater the results are in good agreement with others and at less than that velocity, a slight deviation is noticed. This may be due to some experimental errors which dominated in very low airflow and pressure drop measurements. DEVELOPMENT OF SOLAR GRAIN DRYER DESIGN EQUATION The results obtained from the experiments using airflow created by induced temperature gradients can be applied to estimating design parameters of solar grain dryers in which airflow through grain bed is by natural convection. Few simplifying assumptions to be made are (a) air density inside the dryer is uniform, (b) the dryer is air tight and the hot air leaves only at the top chimney outlet, (c) resistance to airflow of the dryer components is negligible because of small airflow rates involved. Airflow due to temperature gradients in a grain dryer can be represented by a functional relationship that, Q = f(h, Ap, Ap,H) [5] Also equation [1] gives the general relationship for airflow through grain beds as, Q = a(ap/h)b [6] where Q airflow through the grain bed, m 3 /s m 2 h grain bed thickness, m AP pressure drop across the grain bed, Pa density difference between hot and cold air, kg/m 3 H = height of hot air column in the dryer, m In the solar dryer mentioned above the pressure differential across the grain bed (AP) is due to the air density gradient (Ap) induced by the temperature difference (AT) between hot air inside the dryer and the ambient air. Therefore, AP = Ap g H. [7] g = acceleration due to gravity, 9.81 m/s 2 For air density calculations in the air temperature range of 25 to 90 C, Jindal and Gunasekaran (1982) have developed the following empirical relationship: p = (T) [8] 604 TRANSACTIONS of the ASAE 1983

5 p = air density, kg/m 3 T = air temperature, C using this information in equation [7] yields, AP = (H-AT) [9] Replacing T' in equation [6] by equation [9] gives, Q = a( AT-H/h)h [10] Substituting the product constant values 'a' and 'b' obtained from the experiments using natural convective airflow and simplifying, equation [10] becomes, Q = 3.81 x 10-5 (AT-H/h) 0 * 87 [11] For a given solar dryer, the above equation can be used to estimate the airflow rate. Alternatively, a solar dryer could be designed if conditions of minimum airflow rate are established first to accomplish drying in a given period. CONCLUSIONS 1. Simple instrumentation and measurement techniques were developed for a wide range of airflow (0.001 to 0.3 m 3 /s-m 2 ) and pressure drop (0.176 to 200 Pa) measurements. 2. The resistance pressure of paddy increases proportionately with the grain bed depth up to 30 cm. 3. The increase in pressure drop is disproportionate with increase in airflow rate. 4. The results in general are in close agreement with those reported earlier in the similar range of airflow rates. 5. Temperature gradients can establish pressure differential across grain beds which could induce movement of air through it. 6. The product constant values obtained in the experiments using airflow due to induced temperature gradients are useful in equations applicable to the design of solar grain dryers in which airflow through grain bed is by natural convection. References 1. Agrawal, K. K. and P. Chand Pressure drop across fixed beds of rough rice. TRANSACTIONS of the ASAE 17(2): Converse, H. H., A. H. Graves and D. S. Chung Transient heat transfer within wheat stored in cylindrical bin. TRANSACTIONS of the ASAE 16(1): Fenton, F. C Storage of grain sorghums. AGRICULTURAL ENGINEERING 22(5): Gunasekaran, S Measurement of low air flow through paddy. Unpublished M. Eng. Thesis, no. AE Asian Institute of Technology, Bangkok, Thailand. 5. Hukill, W. V Grain cooling by air. AGRICULTURAL ENGINEERING 34(7): Husain, A. and T. P. Ojha Resistance to the passage of air through rough rice. Journal of Agricultural Engineering Research 14(4): Jindal, V. K. and S. Gunasekaran Estimating air flow and drying rate due to natural convection in solar rice dryers. Renewable Energy Review Journal 4(2): Johnson, H. K Cooling stored grain by aeration. AGRICULTURAL ENGINEERING 37(4): McHattie, L. E The transistor as an anemometer. Journal of Physics E: Scientific Instruments 12(7): Muir, W. E Temperature and moisture in grain storage, p In: R. N. Sinha and W. E. Muir (eds.). Grain storage: Part of system. The AVI Pub. Co., Westport, CT. 11. Shedd, C. K Some new data on resistance of grains to air flows. AGRICULTURAL ENGINEERING 32(9): Sheldon, W. H., C. W. Hall and J. K. Wang Resistance of shelled corn and wheat to low air flows. TRANSACTIONS of the ASAE 3(2): Stewart, J. A Moisture migration during storage of preserved high moisture grains. TRANSACTIONS of the ASAE 18(2): Wieneke, F Solar-aeration drying of some tropical products, p In: G. Singh and J. J. DeGoed (eds.). Proceedings of the International Conference on Rural Development Technology: An Integrated Approach. Asian Institute of Technology, Bangkok, Thailand TRANSACTIONS of the ASAE 605