Engineering

Solar air heaterEnergy is a crucial input in the process of economic, social and industrial development of any nation. During past several decades, energy demand of the world has been increasing continuously at an alarming rate due to increase in population, industrialization, transportation etc. Continuous use of fossil fuels have resulted energy crisis and environment degradation at global level. On the many alternatives, solar energy is an important renewable energy resource that has the potential of fulfilling all energy needs.

Some important applications of solar energy are solar water heating, solar space heating/cooling, solar cooking, solar crop drying, solar power generation etc. Simplest method to utilize solar radiation is to convert it into thermal energy for heating applications by using solar collectors. Solar air heaters because of their inherent simplicity are cheap and are used for many domestic and commercial applications like space heating, crop drying, wood seasoning etc. Thermal energy of hot air flowing from solar air heater can be stored to use the same in absence of solar radiation for various applications.

 

A solar air heater is a type of energy collector in which energy from the sun is captured by an absorbing medium and used to heat air. Solar air heater has myriad of uses and applications. The main applications of solar air heater are drying of crops, seasoning of timber, space heating, fresh air ventilation etc. A solar air heater is very simple in design and requires little maintenance.

 

Geometry Creation:

Solar air heater duct having V-shaped perforated blocks has been considered for CFD investigation (Fig. 1). The flow cross-section of the duct is 750mm × 150mm and duct length is 2000mm, which is divided into three sections, an entrance section, a test section and an exit section.

V shaped solar heater geometry

Meshing:

Geometry is meshed in Ansys with following parameters:

Mesh Type: Hexa Dominant, Elements: 159070, Nodes: 31315  further the name creation for the geometry was done as: Inlet, Outlet, Sidewall 1, Sidewall 2, and absorber plate.

Solar heater meshed model

Boundary Condition:

Discrete Transfer Radiation Model:

The main assumption of the DTRM is that the radiation leaving the surface element in a certain range of solid angles can be approximated by a single ray. This section provides details about the equations used in the DTRM.

The DTRM Equations:

The equation for the change of radiant intensity dl along a path ds can be written as:

DTRM Equation

 

where,
a= gas absorption coefficient
l= intensity
T= gas local temperature
σ = Stefan-Boltzmann constant

Hemispherical Solid with angles

Clustering:

DTRM is computationally very expensive when there are too many surfaces to trace rays from and too many volumes crossed by the rays. To reduce the computational time, the number of radiating surfaces and absorbing cells is reduced by clustering surfaces and cells into surface and volume “clusters”. The volume clusters are formed by starting from a cell and simply adding its neighbors and their neighbors until a specified number of cells per volume cluster is collected. Similarly, surface clusters are made by starting from a face and adding its neighbors and their neighbors until a specified number of faces per surface cluster is collected.

The incident radiation flux, Qin, and the volume sources are calculated for the surface and volume clusters respectively. These values are then distributed to the faces and cells in the clusters to calculate the wall and cell temperatures. Since the radiation source terms are highly nonlinear (proportional to the fourth power of temperature), care must be taken to calculate the average temperatures of surface and volume clusters and distribute the flux and source terms appropriately among the faces and cells forming the clusters.

The surface and volume cluster temperatures are obtained by area and volume averaging as shown in the following equations:

surface volume cluster temperature

Where Tsc and Tvc are the temperatures of the surface and volume clusters respectively, Af and Tf are the area and temperature of face f, and Vc and Tc are the volume and temperature of cell C. The summations are carried over all faces of a surface cluster and all cells of a volume cluster.

Solar Calculator Parameter for Morning:

solar calculator parameter Morning

Solar Calculator Parameter for Noon:

solar calculator parameter Noon

Solar Calculator Parameter for Evening:

solar calculator parameter Evening

Results

Case 1: Morning

Temperature Contour along length of the Geometry:

Results Morning 1

Fig. 4: Temperature distribution at inlet, mid plane and outlet.

Results Morning 2

 Fig. 5: Temperature distribution at XY Plane.

Results Morning 3

Fig. 6: Pressure distribution at XY Plane.

Results Morning 4

Fig. 7: Velocity distribution at XY Plane.

Case 2: Noon

Temperature Contour along length of the Geometry:

Results Noon 1

 Fig. 8: Temperature distribution at inlet, mid plane and outlet.

Results Noon 2

Fig. 9: Temperature distribution at XY Plane.

Results Noon 3Fig. 10: Pressure distribution at XY Plane.

Results Noon 4

Fig. 11: Velocity distribution at XY Plane.

Case 3: Evening

Temperature Contour along length of the Geometry:

Results Evening 1

 Fig. 12: Temperature distribution at inlet, mid plane and outlet.

Results Evening 2

Fig. 13: Temperature distribution at XY Plane.

Results Evening 3

Fig. 14: Pressure distribution at XY Plane.

Results Evening 4

Fig. 15: Velocity distribution at XY Plane.