THE EFFECT OF THE WATER FLOW RATE INTO THERMAL. STRATIFICATION IN HOT WATER STORAGE TANKS

Necdet ALTUNTOP1, Mevlut ARSLAN1, Veysel OZCEYHAN1,

1 Dept. of Mechanical Engineering, Erciyes University, 38039 Kayseri, Turkey

Abstract: The numerical analysis of the effects of water flow rate into the thermal stratifications in the cylindrical hot water storage tank has been presented. The obstacle, has dimensions f/H=0.133 and g/D=0,2 and has a gap at the center of, has also replaced in the tank. This obstacle has supplied the thermal stratification between cold (at the bottom) and hot (at the top) water. Because it prevents mixing of two fluids. The flow rate has changed for both tanks with and without obstacles in order to analyze the effect of the volume flow rate. The temperature distributions are also presented for both types. The temperature distributions of the cold water in, hot water out and the temperature differences of the water going to and coming from the collector have been presented into the graph. The temperature distributions of both tank types have been compared to supply as high as possible hot water to usage and as low as possible to heater. At result, The best thermal stratification has obtained with Vk=0,8 m/s. The tank type has obstacle has better performance into thermal stratification compared to the smooth tank.

1. Introduction

Water has widely used in domestic and international area fro solar energy storage as sensible heat. In storage units, when the hot water is being used, the cold water comes instead of the hot water from the main lines and these two waters would be mixed in the tank and the water temperature would decrease drastically. In this study, the obstacle has placed to prevent this disadvantages and the effect of the volume flow arte into the thermal stratification has also investigated.

There is some previous analysis about these subjects as; ALIZADEH, has investigated the thermal behavior of a horizontal cylindrical storage tank both experimentally and numerically. He used one dimensional the Turbulent Mixing Model and Displacement Mixing Model in numerical calculations. He has used some models to prevent unsteady behavior of the vertical temperature distributions [1].

AL-NIMR has solved and presented some mathematical models to determine the effect of the different design parameters on the thermal stratification within the tank and the time required by tank to supply water within a specified outlet temperature [2].

MISRA has analyzed the thermal stratification both theoretically and experimentally in hot water storage tank for the thermo siphon effect in solar water heating systems. He has given the analytical expressions to obtain temperature distributions in the tank. He has also given the diagrams depends on the time to present conductive heat transfer between the layers in the storage tank [3].

HELVA ad MOBARAK have investigated the effect of the amount of the hot water using into the temperature distribution in solar water heating storage tank [4].

HARIHARAN and BADRINARAYANA have analyzed the thermal stratifications numerically
and experimentally in the hot water storage tank. They have studied the effect of surrounding and operating conditions into the thermal stratifications. They have observed that stratification improves with increasing AT and water flow rates [5].

HAHNE and CHEN have studied numerically about the flow and heat transfer characteristics in a cylindrical hot water store. They have used the storage efficiency to obtain thermal stratification. They have found that the increase of the Richardson and Peclect number has an effect that increases the storage efficiency [6].

Mo and MIYATAKE have carried out the transient numerical analysis for the thermal stratifications in the storage tanks. They have used turbulence model (k-s model). They have presented the effect of exchange cold water with hot water into the thermal stratifications [7].

EAMES and NORTON have investigated the effect of the tank geometry into the thermal stratification for sensible heat storage for low Reynolds number. They have presented the effect of inlet and outlet port locations on store performance [8].

In this study, the effect of the using different volume flow rate of the water for obtaining higher thermal stratifications has been analyzed numerically. One obstacle is placed in the cylindrical tank to get the best performance for thermal stratifications inside the tank between all investigated volume flow rates. The water has been used as fluid. The flow type has assumed as turbulent. There would be vortexes in the tank. Because two different fluids are mixed in the tank and they are hitting each other towards to the wall and obstacle surfaces. Therefore, calculations are carried out by using wall functions method and standard k-s turbulent model.

The main difference of this work is that the thermal stratification is supplied by using the obstacle. This obstacle has prevented to mixing of two different fluids to supply higher thermal stratification. The used obstacle has hole at the center. The different volume flow rate is used to analyze the effect of using this obstacle. The previous works has two dimensional calculations but, this present study has carried out by three dimensional.

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