Design of an Indoor Setup for Experimental Investigation of Thermosiphoning Heat Transfer Using Water and Nanofluid for Application in Compound Parabolic Solar Collectors ^†

Abstract

The world is moving towards renewable energy sources because of fossil fuel depletion and its adverse environmental impacts. To study the thermosiphoning process using water and nanofluids at different angles of receiver tubes, an indoor experimental setup was designed. The maximum flow rate achieved at a 35° angle was 6.30 mL/s and the maximum outlet temperature achieved was 82.8 °C at a 45° angle using water. The flow rate achieved using Al₂O₃ nanofluid was 8.20 mL/s. The results show that the time to achieve the thermosiphoning was greatly reduced with an enhanced flow rate of 30.1% using nanofluids as compared with water.

1. Introduction

In recent decades, many advancements have been made in the field of solar energy technology. Parabolic collectors are the type of solar collectors used to concentrate sun rays on an absorber tube [1]. The acceptance angle of the compound parabolic collector determines the maximum number of sun rays to be concentrated [2].The concentrated light is used to heat the liquid flowing through it. The circulation of fluid in the tubes takes place using an active system that requires a continuous power source. However, thermosiphoning is a passive system used to circulate the fluid using natural convection [3]. The main aim of this research is to design an indoor setup to investigate the use of thermosiphoning for the purpose of pumpless water flow in a compound parabolic collector. The numerical results use the Boussinesq approximation method for incompressible fluids [4]. Boussinesq approximation suggests that the variation in all of the fluid properties other than density is ignored. The work accomplished in this research is the design and fabrication of an indoor setup for experimental thermosiphoning results. Using the experimental results, the flow rate, volume accumulated, and temperatures were calculated. The use of thermosiphoning in solar water heaters using the flat plate solar collectors has already been implemented and is a mature technology [5]. However, thermosiphoning has not yet been accomplished in compound parabolic solar collectors. There is a considerable research gap in achieving thermosiphoning in the case of compound parabolic collectors. This research paper is a contribution to achieving passive fluid flow in a compound parabolic solar collector through the principle of thermosiphoning. This study also focuses on using Al₂O₃-based nanofluids with enhanced thermal properties as working fluids. This research has not been carried out previously and very little literature reviews are available on it.

2. Methodology

2.1. Simulation Analysis

Complete setup and analysis was carried out using ANSYS 2021 R2. This is the free convection heat transfer phenomena, where the motion of fluids is caused by the buoyancy forces arising from variation in the density of fluid with the temperature. This is why we used the Boussinesq method, which is a density-based solver used to achieve thermosiphoning. The geometry, meshing, and boundary conditions are shown in Figure 1.

2.2. Indoor Experimentation

The experimentation involved achieving thermosiphoning using water in an indoor designed setup. The heater was made using an induction coil that heats the pipe placed inside of it. The whole setup of the indoor system consists of a heater, copper pipe, water tank, and K-type thermocouple. The schematic diagram and indoor setup diagram are shown in Figure 2.

Readings were taken at different angles of inclination. The copper pipe is inside the heating source to gain maximum heat flux through the heater. The heater is made in a way that maximum heat flux reaches the copper tube. The K-type thermocouple is used to measure the temperature at the inlet and outlet of the copper pipe.

3. Results and Discussion

The theoretical results are from the simulation of the pipe on the ANSYS setup 2021 R2. The energy model was used with a viscous laminar type of fluid. The gravity and buoyancy effect was also added into the simulation. The Boussinesq approximation method was used for defining the values of density, specific heat capacity (C_p), thermal conductivity, and viscosity of the fluid with the change in the temperature of the fluid. The simulation results for temperature and velocity are shown in Figure 3.

The temperature and velocity distribution diagrams show that the outlet temperature obtained was 379 K, whereas the inlet temperature was 300 K. At the start of the thermosiphoning, the whirling phenomena of fluid occurred, then the continuous flow of fluid was obtained. The maximum outlet velocity obtained was about 0.011 m/s. As these numerical results are close to the experimental results, this shows that, with the increase in the flux value, the outlet temperature and velocity contours also increase. In the experimental setup, three cases of receiver tubes at different angles with water as working fluid are studied. The parameters observed during the experimentation were flow rate, inlet temperature, and outlet temperature. All of the parameters are shown in

Table 1. Indoor setup specifications and the experimental results.

	Receiver Tube Angles
	35°	40°	45°
Receiver tube length (in)	21	21	21
Tube diameter (in)	0.63	0.63	0.63
Flow rate achieved (mL/s)	6.30	5.92	4.97
Inlet temperature (°C)	28.5	27	28
Outlet temperature (°C)	76.1	74.9	80.8

below.

The results show that the thermosiphoning starts at different temperatures for all inclination angles. At a 35° angle of the receiver tube, the thermosiphoning starts at a temperature of 54.5 °C, while thermosiphoning start at a slightly higher temperature of 57 °C at an angle of 40°. The longest time and temperature to achieve thermosiphoning took place at 45°, which was 60 °C. This is because a larger temperature difference is required to cause the buoyancy effect. The four cases use a tube diameter of 0.63 in and length of 21 in. The results of the thermosiphoning are shown in Figure 4.

The water-based Al₂O₃ nanofluid with a 0.05% concentration of nanoparticles shows an increased flow rate of 8.2 mL/s at a 35° angle of the receiver tube. The graphs in Figure 5 show that the thermosiphoning took less time and a lower temperature to achieve thermosiphoning as compared with water. The comparison of the numerical and simulation results is also shown in Figure 5 for the case of water as working fluid.

The maximum flow rate of 6.30 mL/s with water was achieved at a receiver tube angle of 35°. The simulation results show a flow rate of 5.8 mL/s. The simulation shows that an outlet temperature of 379 K was achieved. The temperature at which thermosiphoning begins varied between 10 min and 12 min depending on the angle of the tube. The results obtained using water-based Al₂O₃ nanofluid show that the thermosiphoning time was reduced to just 8 min at a 35° angle. The flow rate achieved was also enhanced by 30% with nanofluids. This difference in time was achieved as a result of the enhanced heat transfer capacity of nanofluids. The results show that water and nanofluids can be employed in compound parabolic collectors for a pumpless water flow if the flow rate requirements are very small.