Parabolic Solar Collector Thermal Performance Using Nano Fluid (Validation)

Paper Description

The present problem simulates heat transfer within a tube of a parabolic solar collector containing water flow. This numerical simulation has been done based on the reference paper “Thermal performance analysis of solar parabolic trough collector using nanofluid as working fluid: A CFD modelling study“, and the results have been compared and validated with the results in the article. In fact, in the current model, there is a tube with a water flow that is exposed to solar energy (radiation).

Behind the tube, there is a parabolic plate that absorbs the solar radiant energy, which is responsible for absorbing the heat energy from the sun’s radiation and then reflecting it. In this case, only the water-flowing pipe is modeled; Thus, the wall of the water pipe is divided into two parts, the upper and lower wall. The upper part of the wall is directly exposed to solar energy and receives heat in the form of heat flux from the sun’s rays; While the lower part of the wall is affected by the reflection energy of the parabolic absorber plates of the collector and receives the energy as a constant heat flux.

Paper Description

Also, the wall of the water pipe is made of aluminum. The inlet water flow to the pipe has a Reynolds of 30,000 and a temperature of 320 K, which according to the relationship between the Reynolds number and the amount of thermophysical properties of the water fluid, the value of the water flow inlet velocity is equal to 0.5024043 m.s-1. Based on the relationships in the paper, it is assumed that the pipe wall has a constant heat flux in both its upper and lower parts; So that for the upper wall, the heat flux is equal to 750 W.m-2 and for the lower wall, the heat flux is equal to 19500 W.m-2.

The main purpose of this simulation is to investigate the Nusselt number.

Parabolic Solar Collector Geometry & Mesh

The present model is designed in three dimensions using Design Modeler software. The geometry of the model consists of a tube which, due to its symmetrical structure, is drawn in a semi-cylindrical shape. The tube consists of two parts, the thin outer layer of which acts as the solid wall of the tube and the inner part which acts as the fluid conduit. The pipe has an internal diameter of 0.06 m and a length of 2 m, which has a thickness of 0.002 m. The following figure shows a view of the geometry.

The meshing of the model has been done using ANSYS Meshing software and the mesh type is structured. The element number is 1475000. The following figure shows the mesh.

Parabolic Solar Collector CFD Simulation

To simulate the present model, several assumptions are considered:

• We perform a pressure-based solver.
• The simulation is steady.
• The gravity effect on the fluid is ignored.

A summary of the defining steps of the problem and its solution is given in the following table:

	Models
Viscous model		k-epsilon
	k-epsilon model	RNG
	near-wall treatment	standard wall function
	Boundary conditions
Inlet		Velocity inlet
	velocity magnitude	0.5024043 m.s-1
	temperature	320 K
Outlet		Pressure outlet
	gauge pressure	0 Pascal
Upper Wall		Wall
	wall motion	stationary wall
	heat flux	750 W.m-2
Down Wall
	wall motion	stationary wall
	heat flux	19500 W.m-2
	Solution Methods
Pressure-velocity coupling		SIMPLE
Spatial discretization	pressure	standard
	momentum	first order upwind
	energy	first order upwind
	turbulent kinetic energy	first order upwind
	turbulent dissipation rate	first order upwind
	Initialization
Initialization method		Standard
	gauge pressure	101325 pascal
	temperature	320 K
	z-velocity	0.5024043 m.s-1
	x-velocity, y-velocity	0 m.s-1

Paper Validation

At the end of the solution process, the value of the Nusselt number in the model is calculated and compared and validated with the Nusselt values in the reference article. The amount of surface Nusselt at the level corresponding to the contact boundary between the fluid and the pipe wall is calculated using the REPORT command. Since, according to the paper, the value of the Nusselt number is obtained in the area of fully developed flow, the present numerical simulation also focuses on the value of the Nusselt number at the end of the pipe where the flow is developed.

The table below shows the amount of Nusselt number on the contact surface between water and pipe, which is obtained in different sections of the pipe and at different distances from the outlet section of the pipe. Then the value of the Nusselt number of the article is obtained according to the diagram of Figure 4 of the article and in the value of Reynolds equal to 30,000 of this figure.

Paper Validation

Comparing the amount of surface Nusselt at different sections at the ending areas of the pipe with the amount of Nusselt in the article indicates that the closer we get to the end of the pipe and the area with the developed flow, the accuracy of the solution and the validation of the present simulation becomes more valid.

Present Simulation	distance to outlet	Nusselt	Paper Work	Nusselt
	0 m to outlet	220.0619		221.3
	0.05 m to outlet	221.5318
	0.1 m to outlet	222.7393
	0.15 m to outlet	224.2536
	0.2 m to outlet	225.5326
	0.25 m to outlet	226.9974
	0.3 m to outlet	228.3515
	0.35 m to outlet	229.7931
	0.4 m to outlet	231.0474
	0.45 m to outlet	232.4048
	0.5 m to outlet	233.7537

Two-dimensional and three-dimensional contours related to pressure, velocity, and temperature are also obtained. Two-dimensional contours are drawn in the symmetrical cross section of the model.

There is a mesh file in this product. By the way, the Training File presents how to solve the problem and extract all desired results.