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Mathematical Modelling for Drying of Tomato in Hybrid Dryer

M. A. Hossain
Farm Machinery and Postharvest Process Engineering Division, Bangladesh Agricultural Research Institute, Gazipur-1701, Bangladesh

K. Gottschalk
Institute for Agricultural Engineering, Potsdam-Bornim, Max-Eyth-Allee, 14469 Potsdam, Germany

B. M. A. Amer
Agricultural Engineering Department, Faculty of Agriculture, Cairo University, Egypt

Abstract/ Executive Summary:

A mathematical model was developed for a concentrative flat plate solar collector coupled with an indirect multirack type hybrid dryer. The model was physically-based, taking into account the heat transfer in the collector and heat and mass transfer in the dryer. One set of equations was developed to predict cover, receiver, and air temperatures in the collector. Another set of partial differential equations was developed to predict the air and product temperatures, air humidity, and moisture content for drying of tomato halves in the hybrid dryer. The first set of equations were solved iteratively and the second set of equations were solved numerically based on an exponential solution over the finite difference grid element using the outlet air conditions of the collector as inlet air conditions of the drying unit. The simulated cover, air, and receiver temperatures in the collector agreed well with the measured temperatures. Good agreements were also found between experimental and simulated air and product temperatures, air relative humidity, and product moisture content in the dryer. This model can be used to provide design data of the solar and hybrid dryer for the drying of tomatoes as well as other fruits and vegetables.

Keywords: Collector, Drying, Model, Hybrid dryer, Tomato

Location: Bangladesh Agricultural Research Institute, Gazipur-1701, Bangladesh

Start Date:

End Date:

Program Area: Postharvest and Agro-processing

Commodity: Tomato

Objectives:

a. To develop a mathematical model for solar drying of tomato using an indirect type forced convection solar dryer, and

b. To validate the model with experimental data.

Methodology:

The computer simulation model involves two components that are used to describe the drying process in a solar dryer. These are: (a) The collector performance, which predicts the air temperature and relative humidity and cover and receiver temperatures at different positions of the collector at different drying times. (b) The thin layer simulation model that predicts the air temperature and humidity and product temperature and moisture content at different times and positions in the dryer. A flat plate collector is designed for applications requiring moderate temperatures not exceeding 100°C. It is relatively cheap and can be easily constructed. It uses both beam and diffused solar radiation and is well suited for the drying of agricultural crops. The collector in this study is a concentrator type with a flat plate reflector. It may be termed as concentrator type flat plate air heater because air is the heating fluid transferring heat energy from collector to dryer by forced convection. Total incident radiation flux on the collector cover is the sum of global radiation (directly falls on the cover) and reflected radiation. The dryer under this study is a fixed bed vertically divided into five layers (trays). There is an air gap between two trays. Also, inlet and out layers have similar air gaps for air entry and air exit. Heated air enters from the bottom and exits from the top of the dryer. During the air flow, it carries moisture from the product from each of the layers. During air flow (drying), product loses moisture to air and air gains moisture from the product. The following assumptions are made for simplification of the model. (1) Air flow is one dimensional, (2) There is no heat flow perpendicular to the direction of air flow, (3) Air properties do not change within the air gaps, (4) Contribution to the energy and moisture balances from the rate of air properties are negligible, (5) Outlet air temperature and relative humidity of the collector is the same as inlet dryer temperature and relative humidity. maximum solar radiation could be captured by the solar collector as well as by the reflector. In the afternoon, after 17:00 hours, the samples in the solar dryer were kept in the dryer, the collector was covered by a reflector, and the control samples were kept in the same place with a perforated cover on it so that air could easily move on and around the samples. Next morning, at 09:00 hours, the cover of the dryer and the cover from the control samples were removed and subjected to drying by solar radiation. A data logger (Almeno 5590, Ahlborn Mess-und Regelungstechnik GmbH, Germany) was used to record the ambient air, collector air, drying air (on different trays), inlet air, and outlet air temperature and relative humidities at 10 minute intervals. Temperatures at different positions of glass cover, copper tube, and absorber plate of the collector were also recorded at 10 minute intervals during the drying period. A solar meter (Solarwatt, GmbH, Germany) was used to measure the global solar radiation and total radiation (global+ reflected from reflector) during the day time drying period. The velocity of drying air was measured with an anemometer (TA-5, Airflow Development Limited, England) at and when required. The moisture content of the tomato sample was measured by drying the samples in an air ventilated oven at 105°C for 24 hours. After completion of drying, the dried tomato was collected, cooled in shade to the ambient temperature, and then sealed in plastic bags.

Source of Information: The Arabian Journal for Science and Engineering, Volume 35, Number 2B, October 2010

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Collector cover, air and receiver temperatures and collector thermal efficiency of the dryer were predicted along the length of the collector and their highest value was found at the end of the collector length. The predicted cover temperature, air temperature, and relative humidity at outlet of the collector, and the receiver temperature and collector thermal efficiency were validated with measured data and found to agree well with the measured data. Air temperature, product temperature, and product moisture content were predicted on different trays along the height of the dryer (along the direction of air flow) and with different drying times and they agreed with the fundamental theories of drying. Good agreement was found between experimental and simulated air temperature, air relative humidity, product temperature, and product moisture content of tomato halves during drying. This model can be used for providing design data for solar and hybrid dryers.

Knowledge Management: Journal