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Air-flow Distribution Study and Performance Analysis of a Natural Convection Solar Dryer

Nicholas Musembi Maundu*, Kosgei Sam Kiptoo, Kiprop Eliud, Dickson Kindole, Yuichi Nakajo

Graduate school of engineering, Ashikaga Institute of Technology, Japan, 268-1 Omae, Ashikaga Tochigi, 326-8558, Japan

Abstract: 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.

Keywords: PIV technique, humidity ratio, performance analysis

Citation: Nicholas Musembi Maundu, Kosgei Sam Kiptoo, Kiprop Eliud, Dickson Kindole, Yuichi Nakajo, Air-flow Distribution Study and Performance Analysis of a Natural Convection Solar Dryer. American Journal of Energy Research, 5 (1). 12-22. January 2017. doi:10.12691/ajer-5-1-2

1. Introduction

Concerns over climate change and food security are among the current major challenges in global perspective. The need to minimize utilization of fossil fuels as a mitigation measure in curbing the extent of greenhouse gas emissions has brought about extensive research on renewable energy applications. Renewable energy does not only contribute to a better environment but also give a strong boost to economy through job opportunities as well as improving food security through preservation systems such as solar dryers.

Post-harvest losses have been a challenge to farmers for many years, they have deprived farmers the much needed returns from the farm output and brought down the potential of agricultural contribution to economy. Post harvesting losses are estimated at 30%-40% of the farm produce and may heighten to 80% under adverse conditions such as bumper harvest period [1]. Solar drying is considered a traditional method and has been used by many farmers in developing and developed countries for many years as a food preservation method, traditionally, sun drying as the leading method of food drying. Solar dryers make use of a closed space which accommodates the product being dried, this gives better and hygienic results. Basically, there are three types of solar dryers namely; direct solar dryers, indirect solar dryers, and mixed solar dryers. Description, mode of operation, merits and demerits of each of the dryers have been discussed by A. A. Elbaii and S. M Shalaby, 2012 [2].

Drying process involves vaporizing water (moisture content) in the product, the latent heat of vaporization must be supplied to increase vapor pressure over the product. During this process, airflow is required to remove vapor away from the product [3]. In indirect solar dryers, the latent heat of vaporization is supplied to drying chamber from the solar collector, the solar collector absorbs the solar energy raising the temperature of the air within the collector while lowering its relative humidity. The hot air then flows by convection through the drying chamber and exit through the chimney by thermo-siphoning effect. Distribution of the air flow across the drying chamber is key in determining the efficiency and uniformity of the dried product.

Most solar dryers make use of the tray systems in the drying chamber making trays an important part of the drying bed. Tray dryers are widely used in a variety of applications because of their simple design and capability to dry products at high volume. The greatest drawback of the tray dryer is uneven drying because of poor airflow distribution in the drying chamber. By understanding the airflow distribution analysis methodology, suitable design approach may be used to improve tray dryer performance, increase quality of dried product and produce uniform drying [4].

A lot of emphasis has been placed on thermal performance analyses with most research being devoid of the effect of airflow distribution in the drying chamber to the quality and the overall efficiency of the drying system. In this work, in addition to thermal performance of indirect solar dryer, airflow distribution across drying chamber has been studied by use of PIV technique. PIV is an effective tool in visual analysis of air flow distribution. In addition to visualization of the air flow distribution across the drying chamber, other parameters such as velocity and turbulent kinetic energy are also analyzable. Unlike computational fluid dynamics (CFD) whose prediction of air flows is based on mathematical modeling and computer simulations in a virtual flow laboratory, PIV technique provides real experimental airflow analysis whose results can easily be translated and understood by solar dryer enthusiast with little or no background knowledge on fluid dynamics. PIV provides quantitative and qualitative description flow phenomena of air using measurements as opposed to quantitative prediction flow phenomena provided by CFD software thus making it a suitable tool for laboratory scale models.

2. Materials and Methods

The authors used an already designed and existing solar dryer, the drying chamber was modified to suit PIV study with most of the other parts of the solar dryer system remaining as per original design. The materials used for this dryer are listed in table as shown in appendix C. The solar dryer collector area was 0.56265m² and has an inlet of 0.00925m².

2.1. Modification of the Existing Solar Dryer - Drying Chamber

This is the only part of the dryer which was modified from the existing natural convection solar dryer. The drying chamber was dimensioned to be 52 cm long , 50 cm wide and 69 cm high. The top, bottom and front were constructed using 1.3 cm thick plywood. The door has been provided at the rear end with hinges and wooden locks. It is constructed using 1.3 cm thick plywood, the edges being lined with rubber seals to enable tight-fitting for better thermal insulation.

The two opposite sides (i.e. right and left) of drying chamber were constructed using transparent PVC material of 1mm thickness and its outer edges reinforced using 4 mm thickness plywood.

Three removable trays of dimensions 47cm by 47 cm were constructed using wire mesh with their four outer edges reinforced using plywood. The slide bars for holding the trays were fixed at distances of 20 cm, 36 cm and 53 cm from the bottom of the chamber.

Hot air enters the drying chamber through inlet at the bottom front and leaves through the chimney at the top back face.

Figure 1 and Figure 2 show the initial design and the modified solar dryer respectively

Figure 1. Existing solar dryer design

Figure 2. Modified Design solar dryer for PIV study

2.2. Basic Performance Analysis Equations

The amount of moisture to be removed from the product, , is determined as [5];

[picture] | (1)

Where; = the amount of moisture to be removed from the product, = initial mass of product to be dried, = the initial moisture content in % wet basis and =the final moisture content % wet basis.

The quantity of heat required to evaporate water is determined as shown in eqn.(2);

[picture] | (2)

where Q = the amount of energy required for the drying process in kJ, = latent heat of vaporization in kJ/kg of water.

The latent heat of vaporization () can be determined using equation given by Youcef-Ali et al (2002) [6];

[picture] | (3)

is the temperature of the product being dried in °C, in this case, average temperature was considered. The total heat energy in kJ required to evaporate water in ideal situation can be determined from eqn.(4) [6,7];

[picture] | (4)

where is the mass flow rate of air in kg/hr, and are final and initial enthalpy of drying air in kJ/kg of air and is the global radiation on the horizontal surface during the drying period (kJ/m²).

Where is the rate of radiation incidence on the absorber surface (W/m²) and thus the total radiation incident to the total area of the collector, can be written as:

[picture] | (5)

The reflected energy from the absorber is given as;

[picture] | (6)

Where is reflection coefficient of the absorber and transmittance of the cover.

The drying rate is proportional to the difference in the moisture content between materials to be dried and the equilibrium moisture content. Average drying rate can be determined from the mass of moisture removed by solar heat and drying time by the equation shown below;

[picture] | (7)

Where is the average drying rate, kg/hr.

The percentage moisture removed from the product and the percentage final moisture content of the dried material can be determined from equations 8 and 9 respectively [8];

[picture] | (8)

Where is the final mass of the dried product in kg, the percentage final moisture content;

[picture] | (9)

The total collected solar Energy falling on the collecter can be obtaineded as;

[picture] | (10)

Efficiency of solar dryer in relation to total heat energy for evaporation and irradiation relates as [8];

[picture] | (11)

Where E is the total useful energy received by the drying air in kJ as obtained from egn. (4). The energy falling on the collector from the sun can be converted to units of energy , (kWh) as;

[picture] | (12)

Air pressure across the drying product bed can be determined by the equation given by Jindal and Gunasekaran (1982);

[picture] | (13)

Where is the pressure head (height of the hot air column from the base of the dryer to the point of air discharge from the dryer), P is the air pressure in Pascals, g is acceleration due to gravity and is the ambient temperature (°C).

3. Experiment Set-up

The experiment was carried out in two stages, under no load condition and under load condition. Camera recorder was used to record high speed videos so as to be used in PIV Flownizer Software for airflow analyses. The recording speed of the camera used was 30 frames per second. The experiment set-up for no load condition was as shown on Figure 3. Under load condition, the same solar dryer was used for normal drying of sliced apples to moisture content acceptable for packaging and storage. This phase did not require Electronic smoker, video recorder or the blue back ground.

3.1. Stage I: No Load Condition

Under this condition, the dryer was run under no load . The dryer was placed outside in the sun in order for the airflow condition to stabilize before the experiment was carried out. For the airflow to actualize, the collector absorber and chimney absorber collects solar irradiance thus raising air temperature and facilitating the airflow through the drying chamber by thermo-siphoning effect. The smoke was introduced through the solar collector inlet. The sides of interest were the two opposite transparent sides of drying chamber. On one of these sides, blue background was created using a blue Styrofoam sheet placed 60 cm from drying chamber, while on the opposite side, high speed camera was set for recording videos. The side with the camera recorder was covered with black clothing in order to provide clarity of the videos being recorded and minimize reflection.

3.2. Stage II: Under Load Condition

Red apples were cut into slices of 2.5 mm, weighed using digital weighing scale, spread evenly on the trays and placed in drying chamber. Humidity and temperature data loggers were set at collector inlet, drying chamber inlet and drying chamber outlet for recording both humidity and temperature at the respective points as shown in Appendix A with recording intervals of 20 minutes.

Thermal imaging camera was used in this phase to give an image of representative temperature distribution within the drying chamber.

This experiment was used to dry the product to moisture content (below M.C of 15%) acceptable for packaging and storage. These results were used for the output performance analysis.

Figure 3. Experimental Set-Up

4. Results

The results obtained were categorized into, PIV airflow distribution results and dryer performance results.

PIV Technique Results of Airflow Distribution.

The experiments were carried out on empty trays, the videos obtained were used to carry out particle image velocimetric measurements.

The results here are for airflow of a section approximately 4 cm above the drying chamber inlet and 2 cm just below the 3rd tray. It was difficult to capture video for the whole drying section at once. Some of the results obtained are as shown from Figure 4 to Figure 8.

Figure 4. Velocity across the chamber

Figure 5. Profile of line velocity(UV) across mid-section

Figure 6. Turbulent Kinetic Energy (TKE)

Figure 7. Correlation coefficient (region between the lower tray and upper tray)

Figure 8. Graph of Correlation Coefficient across the drying chamber

Figure 9. Graph of Turbulent Kinetic Energy across the section (between tray 2 and 3)

Figure 10. Graph of standard deviation of velocity within frames across the section

Loaded tray/Dryer Performance Results

For the performance analysis, sliced fresh apples were dried for 8 hours 40 minutes. Some of the results obtained are graphically represented here. Thermal imaging camera results obtained during this session were as shown in Figure 11.

Figure 11. Thermal image showing temperature variations across 4 different sections

The average irradiance was 525.29 W/m². The graph of irradiance against time was plotted as in Figure 12.

Figure 12. A graph of Irradiance (W/m²) against time of drying

Figure 13. A graph of Temperature and Relative humidity against time of drying

The results for temperature (in degrees Celsius), relative humidity (%) against time of the day was plotted as in Figure 13.

For this study, T1, T2, T3 are temperatures at collector inlet, drying chamber inlet and drying chamber outlet respectively, H1, H2 and H3 are relative humidity at collector inlet, drying chamber inlet and drying chamber outlet respectively. The results of other useful values for performance analysis are summarized as shown on Table 1.

Table 1. Summary of calculated values

The humidity ratios at drying chamber inlet (HR2) and drying chamber outlet (HR3) were also obtained. Graph of humidity ratio against time obtained from the results is as shown in Figure 14.

The dried product in the chamber was as shown in Appendix B.

Figure 14. A graph of Humidity Ratio(HR) against time of drying

5. Discussion and Conclusion

Airflow velocity distribution in the drying chamber was captured by Particle Image Velocimetry as shown in Figure 4. The distribution flow is fairly good with a high correlation coefficient as shown in Figure 7 and Figure 8. The correlation coefficient can be estimated to an average of 0.9, this implies a fair flow distribution. Imagery results for Turbulent Kinetic Energy indicates an average of 4.0 x 10-5 m²/s² which implies fairly smooth airflow a characteristic of natural convection flow solar dryer.

To capture the temperature distribution, portable thermal imaging camera was used during actual drying time, the results shown in Figure 11 indicate a variation of 1.4°C in average temperatures between four sections categorized from the bottom of the drying chamber as Line1, Line 2, Line 3 and Line 4.

In order to test the solar dryer performance and relate the airflow study results to actual experiment data, fresh apples were dried and the results obtained during drying process are as shown in Figure 12 to Figure 14. Figure 12 shows the irradiance obtained at intervals of 20 minutes during drying period, the average irradiance was 525.29 W/m². Figure 13. shows a graph of temperatures and relative humidity at the collector inlet, drying chamber inlet and drying chamber outlet. From this graph, of great significance is the falling rate of relative humidity at the drying chamber outlet as the drying proceeds, this may be occasioned by the high relative humidity in the morning as well as high moisture extraction from the product during the initial stage of drying. Humidity ratios for the air at the drying chamber inlet and outlet were obtained by use of psychrometric chart and graph plotted as shown in Figure 14. The dropping humidity ratio of the air from the drying chamber can be related to the reducing rate of the moisture removal from the product as the % moisture content falls.

A summary of drying performance analysis results was tabulated as shown in Table 1. The prototype solar dryer had an overall useful efficiency of 16.47% after drying apples from moisture content of 86% (wet basis) to 13.74% (wet basis).

The objective of the study was achieved with a natural convection solar dryer suitable for mid-latitude application having been designed, fabricated and its performance evaluation studied. PIV technique was successfully applied for the airflow distribution and flow visualization. Thermal imaging was found useful in providing an idea on temperature distribution within the drying chamber. There was fairly uniform distribution of air across the trays from PIV study and that conformed to the results from experiments indicating uniform drying of the product across the trays.

The design for mid-latitude applications can easily be replicated elsewhere in the world at low cost, products can be dried in one day and the dryer has capability to dry products at commercial scale.

Acknowledgements

We acknowledge the support given by the management team of Ashikaga Institute of Technology more so the technical assistant and moral support from the teaching staff and post graduate students in Renewable Energy and Environmental Engineering Division.

References

Appendices

Appendix A: Solar Dryer set-up showing data logger (relative humidity and temperature) positions

Appendix A

Appendix B: Dried apples in drying chamber.

Appendix B

Appendix C: Material used to fabricate the dryer.

Summary of Materials used for Construction of the solar dryer

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