Abstract:
The design of a water based thermal store for use in a domestic
central heating system has been investigated theoretically,
experimentally and numerically. The transient operation of the
store during both the space heating and domestic hot-water
modes of operation have been investigated separately.
Heat transfer correlations in terms of Nusselt and Rayleigh
numbers have been developed in order to predict the natural
convection heat transfer coefficient for the outside surface of
the horizontal axis finned tube heat exchanger coil located
within the store. These heat-transfer correlations can predict
the value of the heat transfer coefficient with an accuracy of
better than 5% and are in good agreement with existing heat
transfer correlations developed for the same geometry of finned
tubes and modes of heat transfer. The effect of the water flow
rate in the heat exchanger coil on the internal heat transfer
coefficient is also investigated. This flow rate should be
above 4 litre/minute to achieve a high rate of heat-transfer
from the wall of the heat exchanger to the water in the pipe.
A detailed investigation of the use of horizontal and vertical
baffles to increase the effectiveness of heat delivery in the
domestic hot water mode has been carried out. Some improvements
can be achieved by the use of a horizontal flat plate located
in the middle of the store. This plate, when correctly sized
enhances stratification and hence improves the effectiveness of
heat recovery. Vertical plate arrangements and a rectangular
duct situated around the upper heat exchanger coil were found
to be ineffective. However, due to an increased velocity of the
water around the heat exchanger, the external heat transfer
coefficient of the heat exchanger was increased by 12%.
The comparison of experimental observations with computer
simulations of the development of the thermocline in the store
during the space heating mode of operation showed the presence
of a jet in the bottom region of the store at the return inlet.
The jet induces a significant amount of mixing in the store
which reduces the effectiveness of heat recovery. Correlations
in terms of Richardson number and effectiveness of heat
delivery have been developed to characterize the effect of this
jet. An inlet arrangement designed to achieve a Richardson
number exceeding 3 significantly reduces the mixing created by
the jet and can increase the amount of heat delivered in the
space heating mode by approximately 5%.
