A Novel Approach of Heat Rate Enhancement in Rectangular Channels with Thin Porous Layer at the Channel Walls

Heat transfer enhancement is a topic of great interest nowadays due to its different applications in industries. A porous material also known as metallic foam plays a major role in heat enhancement at the expense of pressure drop. The flow in channels demonstrates the usefulness of this technology in heat extraction. In our current study, a porous strip attached to the walls of the channels is proposed as an alternative for heat enhancement. The thickness of the porous strip was varied for different Reynolds numbers. By maintaining a laminar regime and using water as a fluid, we determined an optimum thickness of porous material leading to the highest performance evaluation criterion. In our current study, with the aspect ratio being the porous strip thickness over the channel width, an aspect ratio of 0.2 is found to be the alternative. A 40% increase in heat enhancement is detected in the presence of a porous strip when compared to a clear channel case for a Reynolds number equal to 200, which improves further as the Reynolds number increases accordingly.

Novel Approach of Heat Rate Enhancement in a Rectangular Channels with Thin Porous Layer at the Channel Walls

Heat enhancement is a topic of great interest nowadays due to its different application in industries. Porous material also known as metallic foam plays a major role in heat enhancement at the expenses of pressure drop. Flow in channels demonstrate the usefulness of this technology in heat extraction. In our current study, a porous strip attached to the channels walls is proposed as an alternative for heat enhancement. The thickness of the porous strip was varied for different Reynolds number. By maintaining laminar regime and using water as fluid, we determined an optimum thickness of porous material leading to the highest performance evaluation criterion. In our current study with the aspect ratio being the porous strip thickness over the channel width, an aspect ratio of 0.2 is found to be the alternative. A 40% increase in heat enhancement is detected in the presence of porous strip when compared to a clear channel case for a Reynolds number equal to 200 and improve further as the Reynolds number increase accordingly.

A Measurement Method of Tensile Strength for Irregular Small Stones

This article studied the drilling loading method for the tensile strength of irregular small-stone, designed the measuring devices, and addressed the measurement problems of irregular small-stone tensile strength. Based on static equilibrium theory, it analyzed the distribution rule of tensile stress in dangerous section, and established the mechanics model of drilling loading method. In considering the impact rules of characteristic destruction size and round-hole size on the hole-edge stress concentration, on this basis it established the theoretical formulas of measuring the tensile strength of brittle materials. It practically measured the failure load of small drilling bluestone, introduced the failure load into the theoretical measurement formula of brittle material tensile strength, and then obtained the tensile strength of stones, while it measured the tensile strength of non-porous strip specimen. The tensile strength of irregular stones measured by the drilling loading method is basically consistent with the tensile strength obtained from the tensile test of non-porous strip specimen, and therefore the drilling loading method is able to reasonably measure the tensile strength of small stones.

EFFECT OF RIBLETS AND SUCTION ON A FLAT PLATE TURBULENT BOUNDARY LAYER

This work presents hot-wire measurements in a flat plate turbulent boundarylayer, subjected to the combination of riblets and suction. The suction is applied through a porous strip for a range of suction rates. The effect of riblets and suction has been quantified through the measurements of mean velocity and Reynolds stresses downstream of the suction strip on the riblets surface. The results of the mean velocity and Reynolds stresses indicate that there is no significant change in the distributions of riblets and smooth wall. However, there exist some changes with the combination of suction and riblets relative to the smooth surface. These changes arise from the interference of suction with the mechanism of the layer. The results suggest that riblets may not alter the effect suction has on the boundary layer structures.

Effects on Topology of a Turbulent Channel Flow Subject to Blowing Through a Porous Strip

An experimental investigation is performed on a fully developed turbulent channel flow with local injection through a porous strip. The Reynolds number based on the channel half-width was set to 5000. In addition to the no blowing data, measurements are made for three different blowing rates σ = 0.22, 0.36 and 0.58 (where σ is the ratio of momentum flux gain due to the blowing and momentum flux of the incoming channel flow). Measurements carried out with hot-wire anemometry reveal that injection strongly affects both the velocity profiles and the turbulence characteristics. The injection decreases the skin friction coefficient and increases all the Reynolds stresses downstream the blowing strip. Moreover, the anisotropic invariant map (A.I.M.) for the Reynolds stress tensor revealed that blowing decreased the anisotropy of the turbulent structure in the near wall region and a decrease in the longitudinal integral length scale was observed when the blowing rate increased. The space time correlation measurements show that injection increases the inclination of the coherent structures in both (x,y) and (x,z) plan.

Turbulence Structures Downstream of a Localized Injection in a Fully Developed Channel Flow

The effects of localized blowing through a porous strip on a turbulent channel were studied experimentally. The measurements were conducted downstream of the porous strip for three blowing rates: 3%, 5%, and 8% (of the velocity at the centerline of the channel). It was found that the injection affects several turbulence parameters. Indeed, blowing decreases the skin friction while it increases the turbulence intensities and the Reynolds stresses. A study of cross-correlations in the streamwise and spanwise direction’s showed that both inclination angles in the (x,y) and (x,z) planes were increased with blowing.