Flow Field Analysis

The numerical analysis carried out on the closed loop thermo-syphon solar water heating system depicts the natural convection phenomena, the distribution of temperature and velocity of working fluid within the model. The natural convection phenomenon happens when the working fluid gets heated and the temperature of the working fluid increases consecutively decreasing its density. Hence, the volume of the working fluid increases. The lower density fluid moves towards the top wall of the riser pipe. For example, increasing the water temperature by 8ºC cause the density to decrease by 2.123kg/m3 [125] Due to the inclination of the riser pipes, the working fluid accelerates along the top wall of the riser pipe and enters the upriser and subsequently into the condenser which is based in the water storage tank.

Figure 5-3 depicts the velocity distribution of the working fluid within the thermo-syphon loop and hot water within the water storage tank at a heat flux corresponding to 15th March under thermal loading condition of weekday. In order to understand the working fluid behaviour within thermo-syphon loop at various heat flux conditions, different times have been chosen on that day. According to the findings, the average velocity of the fluid increases until midday (12 O‘clock), achieving a value of 0.0077m/s, since the maximum heat flux is emitted at that time. Subsequently, the average velocity starts to decrease to 0.0057m/s at 16 O‘clock. The working fluid attains the highest velocity at the upriser and downcomer, and the average velocity of the

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BASELINE MODEL

working fluid is measured to be 0.0233m/s, which is equal to the sum of velocities for all riser pipes. There is no significant difference in the average velocity of working fluid, and the approximated value of working fluid in each riser pipe is 0.00466m/s. The total average velocity of the working fluid in the thermo-syphon is measured. Moreover, since the diameter of the condenser is bigger than the diameter of the upriser and downcomer, it can be seen that the velocity in the condenser is lower as compared to the upriser and downcomer. Similar analogy can be applied for bends in the upriser and downcomer where cross sectional area increases thus decreasing the velocity.

(a) (b)

(c) (d)

Figure 5-3 Flow velocity variations of the working fluid within thermo-syphon loop and water within the storage tank on 15th March under thermal weekday loading for (a) 10 O‘clock (b) 12

Figure 5-4 further depicts the natural convection phenomena occurring in the thermo-syphon model considered in the present study, which is represented in a form of temperature distribution in the thermo-syphon system. As mentioned earlier, the working fluid heats up in the riser pipe and moves upward in the pipe to the upriser. Along the riser pipe, more thermal energy of the solar rays is transferred to the working fluid increasing the internal energy and temperature further. It can be seen that the highest temperature of the working fluid is observed at the riser pipes and upriser junction while the lowest temperature of the working fluid is observed at downcomer. In this case, the maximum and minimum average temperatures are measured to be 80.20ºC and 54.74ºC respectively for 12 O‘clock heat flux condition. Whilst regarding water within the storage tank, it can be clearly seen that the maximum temperature of water is 29.85ºC at upper section of the storage tank and the minimum temperature of water is 15ºC at the bottom section of the storage tank, where the inlet is located. Furthermore, heat of the working fluid is accumulated along the top wall of the condenser and the water within the storage tank.

It would be prudent at this point to present the mass balance in the thermo-syphon loop in order to analyse its performance. From the current setup and configuration, it is expected not to have any recirculation in the riser pipe. The only way to recirculate the flow is through the recirculating pipe. Which refers that the velocity direction within the riser pipe and the recirculation pipe will be opposite and the total mass flow through all the riser pipes should equate to the mass flow through the recirculating as it is shown in Figure 5-5. Figure 5-5 represents the mass balance in the riser pipes and the recirculating pipe. It can be seen that the sum of the mass flow rates of the working fluid passing at any cross-section of the riser pipes is equal to the mass flow rate of the working fluid through the cross section of the recirculating pipe. Hence, the mass is balanced in the thermo-syphon loop considered in the present study.

BASELINE MODEL

(c) (d)

Figure 5-4 Static temperature distributions of the working fluid within thermo-syphon loop and water within the storage tank on 15th March at midday under thermal weekday loading for (a) 10

O‘clock (b) 12 O‘clock (c) 14 O‘clock and (d) 16 O‘clock