Nicholas Musembi Maundu^1,, Kosgei Sam Kiptoo¹, Kiprop Eliud¹, Dickson Kindole¹, Yuichi Nakajo²

In this research work, an indirect natural convection flow updraft solar dryer was studied with objectives to; study air-flow distribution using Particle Image Velocimetry (PIV) technique, analyze relative humidity and humidity ratio variations in relation to temperature, visualize temperature distribution on drying surface and carry out performance analysis of solar dryer. An already existing dryer was modified in order to suit PIV study experiments. The dryer consisted mainly of; hinged flexible angled solar collector, drying chamber consisting of three drying trays and updraft chimney with metallic absorber. The smoke was introduced, by use of an electronic smoker for visualization of distribution of airflow in the drying chamber, and recorded via high speed camera of 30 fps for the purpose of analysis by use of PIV software. Thermal vision camera was used to capture temperature variation images which indicated highest variance of 1.4°C from the mean of the four line sections. For the general performance analysis, experiments were carried out by drying 2.5 mm sliced apples. Temperature and relative humidity of the airflow were taken at the collector inlet, drying chamber inlet and drying chamber outlet using data loggers. Airflow inlet to solar collector and from drying chamber were determined using hot wire anemometers, irradiance (W/m²) was measured using radiation meter. PIV results were achieved indicating fair airflow distribution across the drying bed with high correlation coefficient average at around 0.9 and low velocity standard deviation of below 0.004 in a frame across the sections. Imagery results for Turbulent Kinetic Energy indicated an average of 4.0 x 10-5 m²/s² which implied a fairly smooth airflow which can be said to be a characteristic of natural convection updraft solar dryer. Experiments conducted on loaded solar dryer showed high uniformity on the dried product across the trays; thus, an indication of good airflow distribution across the drying trays corresponding to PIV results obtained. Fresh apples of moisture content 86% (wet basis) were dried to moisture content of 13.74% (wet basis) within 8 hours 40 minutes; the dryer achieved overall efficiency of 16.49 % at an average horizontal irradiance of 525.29 W/m². The dried apples were found to have good texture, color and taste.

PIV technique, humidity ratio, performance analysis