Study the Effect of Heat Losses on the Thermal Stratification in Cylindrical Hot Water Storage Tank

Abstract

Storage tank with different hot water inlet flow rate (4, 6, 8 and 10) L/min and with insulated and uninsulated wall surface is studied experimentally and numerically. The objective of the investigation is to show the effect of heat losses from the tank to the surrounding on the thermal stratification within the water storage tanks and to evaluate the possible enhancement in the thermal performance of solar storage systems. A water storage tank system is designed for practical tests for different modes of operation (charging mode, discharging mode, charging & discharging mode simultaneously) in experimental part. The temperature inside the tank is measured experimentally at three different levels. It was shown that the thermal stratification inside the tanks depends differently on the flow rate, the heat losses as well as the initial temperature in the tank. A 3D computational fluid dynamic (CFD) model using the commercial software package CFX 15 was used to simulate the temperature distribution along the tank and to evaluate the stratification number. The model consist from vertical cylindrical tank of high (1.5m) and inlet diameter of (0.55m). The results shown that the heat losses from the storage tank to the ambient is a main ingredient in retrogression of the thermal stratification within un-insulated tank and the best thermal stratification can be obtained for charging mode of operation. A numerical comparison between two cases (insulated and uninsulated wall surface) was carried out and a performance parameter the degree of stratification was calculated for each case. The experimental and numerical results show reasonable agreement. The average deviation between the experimental and numerical degree of stratification are in order of about (4%).