Flow Characteristic and Heat Transfer for Non-Newtonian Nanofluid in Rectangular Microchannels with Teardrop Dimples/Protrusions

Porous cavity technology is one of the effective ways to improve local flow structures and thus the overall heat transfer of heat exchanging devices. In the present investigation, the flow characteristics and heat transfer in a microchannel heat sink with teardrop dimples/protrusions are studied with a numerical method. The working substances are Al2O3-water nanofluids, which are defined by power-law shear-thinning model. The relative depth and positive eccentricity of dimples/protrusions arranged in the microchannels are 0.2 and 0.3 respectively. The inlet velocity varies in the range of 1.41 m⋅s−1to 8.69 m⋅s−1and the volume fraction ranges from 0.5% to 3.5%. The effects of the flow and heat transfer characteristics are investigated by analyzing the limiting streamlines structures and temperature distributions. The overall thermal performance is evaluated by parameters of Fanning friction factor, Nusselt number and thermal performance. It is shown that the combination of teardrop dimple/protrusion structure and Al2O3-water nanofluids could effectively strengthen heat transfer with low pressure loss. Moreover, in order to obtain the best overall thermal performance, working substances with volume faction of 3.5% is preferred for the proposed microchannel structure.

A new wind tunnel for the study of pressure-induced separating and reattaching flows

The design, construction, and validation of a new academic wind tunnel is described in detail. The wind tunnel is of a classical, blow-down type and generates a pressure-induced, turbulent separation bubble on a flat test surface by a combination of adverse and favorable pressure gradients. The Reynolds number, based on momentum thickness just upstream of separation, is Reθ≃ 5,000 at a free-stream velocity ofUref= 25ms−1. The length of the separation bubble is estimated at 0°42 ± 0°02m by three different methods. Results of a numerical simulation demonstrate the absence of flow separation in the wind-tunnel contraction. This results in a turbulence level of about 0·05% in the test section. Oil-film visualisation experiments show that the flow near the wall is strongly three-dimensional in the recirculating region and that the topology of the limiting streamlines is consistent with experiments performed on configurations with fixed separation. Finally, spatial variations of the forward-flow fraction have been documented using a thermal-tuft probe and are shown to compare well with the results of the oil-film visualisation.

A Study on the Non-Gaussian Property by the Method of Limiting Streamline

Under wind action, the property of non-Gaussian wind pressure distribution is very important for determination of peak factor, simulation of fluctuating wind pressure and design of cladding and components. Since the distribution of non-Gaussian zone is highly irregular based on the statistical method, any effort on giving regular non-Gaussian zone regions cant reflect the real non-Gaussian property. Considering the fact that non-Gaussian property was induced by flow separation and generally the movement of vortex showed the character of time averaged steady, taking a typical low-rise building as the example, the distribution of limiting streamlines and the theory of flow separation was applied in the research. Results show that the distribution of limiting streamlines has a close relationship with the intensity of non-Gaussian property and can be used as an intuitionistic tool in the research of identification and mechanism for non-Gaussian properties.

Three-Dimensional Separations in Axial Compressors

Flow separations in the corner regions of blade passages are common. The separations are three dimensional and have quite different properties from the two-dimensional separations that are considered in elementary courses of fluid mechanics. In particular, the consequences for the flow may be less severe than the two-dimensional separation. This paper describes the nature of three-dimensional (3D) separation and addresses the way in which topological rules, based on a linear treatment of the Navier-Stokes equations, can predict properties of the limiting streamlines, including the singularities which form. The paper shows measurements of the flow field in a linear cascade of compressor blades and compares these to the results of 3D computational fluid dynamics (CFD). For corners without tip clearance, the presence of three-dimensional separation appears to be universal, and the challenge for the designer is to limit the loss and blockage produced. The CFD appears capable of predicting this.

Three-Dimensional Separations in Axial Compressors

Flow separations in the corner regions of blade passages are common. The separations are three dimensional and have quite different properties from the two-dimensional separations that are considered in elementary courses of fluid mechanics. In particular the consequences for the flow may be less severe than the two-dimensional separation. This paper describes the nature of three-dimensional separation and addresses the way in which topological rules, based on a linear treatment of the Navier-Stokes equations, can predict properties of the limiting streamlines, including the singularities which form. The paper shows measurements of the flow field in a linear cascade of compressor blades and compares these with the results of 3D CFD. For corners without tip clearance, the presence of three-dimensional separation appears to be universal and the challenge for the designer is to limit the loss and blockage produced. The CFD appears capable of predicting this.

Numerical Prediction of Flow and Heat Transfer in a Rectangular Channel With a Built-in Circular Tube

Numerical investigation of flow and heat transfer in a rectangular duct with a built-in circular tube has been carried out for a Reynolds number of 1000 and blockage ratio of 0.44. Since the heat transfer in the duct is dictated by the flow structure, the present study is directed toward characterization of the flow structure. To this end, the topological theory shows the promise of becoming a powerful tool for the study of the flow structure. Computations show helical vortex tubes in the wake and existence of horseshoe vortices. The w component of velocity is surprisingly large in front and in the near wake of the tube. The limiting streamlines on the tube and the bottom-plate reveal a complex flow field. The separation lines as well as singularity (saddle and nodal) points have been investigated. The iso-Nusselt number contours and the span-averaged Nusselt number in the flow passage shed light on the heat transfer performance in the duct.