Prediction of bulk potato temperature during free natural ventilation storage

Abstract

The broad objective of this research was to predict bulk potato temperatures at vanous locations and at different times of storage in boxes under free natural ventilation using both theoretical and physical experimental approaches. Mathematical modelling and simulation were done using available empirical and static models namely Burton (1989) and Nyaanga (1991) after the necessary modifications and a semi-dynamic and theoretical model [named Computational Thermal Prediction (CTP)] that was developed during the research. Experiments using potato storage boxes, thermocouple sensors and data loggers were conducted. The Computational Thermal Prediction (CTP) semi-dynamic model was developed as a compromise between the specific, static and less precise empirical prediction expressions and the generalisable, very precise and highly dynamic models such as Computational Fluid Dynamics (CFD/CFX by AEA, 1996) which demand a lot of Central Processing Unit (CPU) memory and time; after the two were found inadequate for free natural ventilation which is neither fully static nor that dynamic as is the case of forced and other forms of controlled potato storage. CTP mathematical relationships involving the various parameters of the storage environment such as temperature; rate and amount of heat of respiration generated by the potatoes into the system and the amount lost by convection; and the physical characteristics of the bulk such as dimensions, porosity and density were developed based on the theories of free natural convection and results from experiments with appropriate assumptions. Finite element method of numerical analysis was used to determine the temperature at various locations (mainly along the vertical axis) within the bulk and at various time intervals. Experiments were conducted on physical models using potato storage boxes and logging the data after being sensed by thermocouples. The data was used to test and verify the mathematical models and determine the temperature variations which the models could not reveal such as lateral temperature gradients. The results of the research indicate that it is possible to predict the bulk potato temperature using empirical mathematical expressions using the modified Burton (1989) and Nyaanga (1991) models; develop a more generalisable Computational Thermal Prediction (CTP) model for the simulation of bulk potato temperatures; and experimentally determine various bulk potato temperatures in free natural ventilation potato storage. The empirical model predictions were not as accurate as the experiments but they gave accurate trends (history) of the bulk potato temperatures with the time of storage. The modified Burton (1989) model shows the temperature gradients in a bulk of potatoes although with slightly higher magnitudes and variations compared to the observed. The Nyaanga (1991) model simulations gave nearly the same values for all the major temperatures within a bulk of potatoes that is, the temperature at the bottom, centre and top were the same. The Computational Thermal Prediction (CTP) simulations were more accurate than the empirical models and the computer program gives a wide range of options. The individual temperature trends, by various CTP options and at the different points of monitoring were the same at a probability of 1% using general linear model. Generally, the CTP predicted values are slightly higher than the observed. The small deviations could have resulted from assumptions and constants used in the mathematical model such as the major modes of heat and air exchange and their quantification. The CTP computer program using user information about the free ventilation potato storage, gives the user information and recommendations about the specified storage system with respect to bulk size, store temperature and time of storage. However, all the simulations showed that the bulk potato temperature varied with store specifications and ambient temperatures as expected.