Numerical studies on the effect of delta-shaped obstacles’ spacing on the heat transfer and pressure drop in v-corrugated channel of solar air heater

Abstract

Solar air heater (SAH) is simple in construction compared to solar water heater. Yet, it is very useful for drying or space heating. Unfortunately, the convective heat transfer between the absorber plate and the air inside the solar air heater is rather low. Some researchers reported that obstacles are able to enhance the heat transfer in a flat plate solar air collector and others found that a v-corrugated absorber plate gives better heat transfer than a flat plate. Only a few research combines these two in a SAH. This paper will describe the combination from other point of view, i.e. the spacing between obstacles. Its spacing possibly will effect the heat transfer and pressure drop of the air flowing across the channel.The first step in numerical study is generating mesh or grid of the air flow inside a v-corrugated channel which was blocked by some delta-shaped obstacles. The mesh was designed three-dimension and not uniform. The mesh are made finer for area near obstacles and walls both for upper and bottom, and then gradually coarser. Grid independency is the next step to be conducted. When the mesh is already independent, the numerical study begins. To validate the numerical model, an indoor experiment was conducted. Turbulent model used was Shear Stress Transport K-ω (SSTK-ω) standard. Having a valid numerical model, the spacing between obstacles was studied numerically. Ratio spacing to height, S/H of obstacles investigated were 0.5; 1; 1.5; and 2.From numerical studies in a v-corrugated duct, it is found that backflow between obstacles and high velocity in the gap between obstacles and absorber plate causes the flow became more turbulent and enhanced the convection heat transfer between the air and the absorber plate. Obstacles placed in a small spacing will increase Nusselt number (convection heat transfer) and friction factor (pressure drop). The Nusselt number enhanced from 27.2 when no obstacle used to 94.2 when obstacles inserted with S/H=0.5. The Nusselt enhanced 3.46 times. The friction factor will increase from 0.0316 at no obstacle to 0.628 at ratio S/H=0.5. The friction factor increased 19.9 times. Efficiency, Nusselt number, and friction factor are decreasing as ratio S/H is increasing. When ratio S/H used is 1 instead of 0.5, Nusselt number enhancement decreased only 1.13%, but friction factor decreased 15.1%. So, sacrificing a small amount of Nusselt number but reducing a significant friction factor is advantageous. The optimal spacing ratio S/H of delta-shaped obstacles inserted in a v-corrugated SAH is one. In other words, the optimal spacing of obstacle equals to its height.