Wenxian Lin

Direct simulation of natural convection cooling in a vertical circular cylinder

Abstract The transient processes of cooling-down and stratifying an initially homogeneous fluid by natural convection in a vertical circular cylinder have been investigated numerically. The transient flow patterns are identified by the visualization of the transient evolving processes in the cylinder. The results show that vigorous flow activities concentrate mainly in the vertical thermal boundary layer along the sidewall and in the horizontal region which is the lower part of the domain where the cold intrusion flow is embedded. The transient flow patterns at the unsteady and quasi-steady stages are analysed, including the activities of the travelling waves in the vertical thermal boundary layer along the sidewall and the cold intrusions in the horizontal region. A scaling analysis is used to characterize the development of the vertical thermal boundary layer on the sidewall and the stratification in the cylinder. It is found that the numerical solutions agree very well with the scaling results.

Direct simulation of weak axisymmetric fountains in a homogeneous fluid

The weak axisymmetric fountain that results from the injection of a dense fluid upwards into a large container of homogeneous fluid of lower density is studied numerically. Using a time-accurate finite volume code, the behaviour of fountains with both a uniform and a parabolic profile of the discharge velocity at the source have been investigated. The evolution of the transient fountain flow has been analysed and two distinct stages of evolution have been identified. The time series of the passage of the fountain front has been presented and the initial, temporary and final characteristic fountain heights have been determined and scaled with the Froude number at the source. At steady state, the final fountain height and the fountain width are found to be the height and horizontal length scales which provide the full parameterization of the fountain flow in the fountain core. The vertical velocity and temperature on the symmetry axis have been scaled with the height scale and an explicit correlation is also obtained for the former. The radial distributions of both the vertical and horizontal velocities in the zone of self similarity in the fountain core at steady state have been scaled with the two length scales and empirical correlations have been obtained.

Natural convection cooling of rectangular and cylindrical containers

Abstract Scalings are obtained for the development of unsteady natural convection, both in rectangular and vertical circular containers, and are validated by comparison with results obtained using direct numerical simulation. It is found that the numerical results agree well with the scalings. Three main stages of flow evolution are identified and the differences at each of these stages between the rectangular and cylindrical geometries are quantified. It is shown that in the flow regimes considered there is a difference in thermal boundary layer thickness; that the horizontal intrusion layer has a uniform thickness for the rectangular flow, but increases towards the symmetry axis for the cylindrical flow; and that the rate of stratification is much faster for the cylindrical geometry.

Low-Reynolds-number fountain behaviour

Experimental evidence for previously unreported fountain behaviour is presented. It has been found that the first unstable mode of a three-dimensional round fountain is a laminar flapping motion that can grow to a circling or multimodal flapping motion. With increasing Froude and Reynolds numbers, fountain behaviour becomes more disorderly, exhibiting a laminar bobbing motion. The transition between steady behaviour, the initial flapping modes and the laminar bobbing flow can be approximately described by a function FrRe2/3 =C. The transition to turbulence occurs at Re > 120, independent of Froude number, and the flow appears to be fully turbulent at Re ≈2000. For Fr > 10 and Re 120, sinuous shear-driven instabilities have been observed in the rising fluid column. For Re 120 these instabilities cause the fountain to intermittently breakdown into turbulent jet-like flow. For Fr 10 buoyancy forces begin to dominate the flow and pulsing behaviour is observed. A regime map of the fountain behaviour for 0.7Fr 100 and 15Re 1900 is presented and the underlying mechanisms for the observed behaviour are proposed. Movies are available with the online version of the paper.

A parametric study on the thermal performance of a solar air collector with a v-groove absorber

In this paper, a parametric study on the thermal performance of a solar air collector with a v-groove absorber has been investigated. In this single-cover collector, the air flowing in the channel formed by the v-groove absorber and the bottom plate—which is flat and insulated—is along the groove, aiming at enhanced heat transfer rate between the air and the absorber by increasing the heat transfer surface area, which is crucial to the improvement of thermal performance of a solar air collector. To quantify the achievable improvements with the v-groove absorber, a flat-plate solar air collector where both the absorbing plate and the bottom plate are flat, is also considered. The thermal performance of these two types of solar air collectors is analyzed and compared under various configurations and operating conditions. The results show that the v-groove collector has considerably superior thermal performance to the flat-plate collector. It is also found that to achieve better thermal performance, it is essential to; use a small size of the v-groove absorber for the v-groove absorber collector and to maintain a small gap between the absorber and the bottom plate for the flat-plate collector; to use selected coatings that have a very high absorptivity of solar radiation but a very small emissivity of thermal radiation on the absorber and glass cover; to maintain an air mass flow rate above 0.1kg/m2s; and to operate the collectors with the inlet fluid temperature close to that of the ambient fluid.

Cooling of a Pr<1 fluid in a rectangular container

The flow behaviour associated with the cooling of an initially quiescent isothermal Newtonian fluid with Prandtl number Pr less than unity in a rectangular container by unsteady natural convection with an imposed lower temperature on vertical sidewalls is investigated by scaling analysis and direct numerical simulation. The flow is dominated by two distinct stages of development, i.e. the boundary-layer development stage adjacent to the sidewall and the subsequent cooling-down stage. The first stage can be further divided into a start-up stage, transitional stage, and steady-state stage. The parameters characterizing the flow behaviour are the boundary-layer thickness, the maximum vertical velocity within the boundary layer, the time for the boundary layer to reach the steady state, the Nusselt number across the sidewall at the boundary layer development stage, the time for the fluid in the container to be fully cooled down, and the average fluid temperature over the whole volume of the container.

A Comparative Study of the Thermal Performances of Cross-Corrugated and V-Groove Solar Air Collectors

In this paper, the thermal performance of cross-corrugated and v-groove solar air collectors has been compared under a wide range of configuration and operating conditions. For cross-corrugated collectors, the air flow channel is formed by the wavelike absorbing plate and the wavelike bottom plate, which are transversely positioned, whereas for v-groove collectors it is formed by the v-groove absorbing plate and the flat bottom plate. The use of the transversely positioned wavelike bottom plate in cross-corrugated collectors is to achieve better thermal performance by enhancing the heat transfer rate inside the air flow channel via augmented turbulence. It is found that cross-corrugated collectors are superior to v‐groove ones for all the configurations and operating conditions considered, confirming that the use of the transversely positioned wavelike bottom plate, in cross-corrugated collectors improves the thermal performance. The results also show that a slender configuration along the air flow direction, a small gap between the absorbing plate and the bottom plate, a selected coating on the absorbing plate and the glass cover that has a very high absorptivity of solar radiation but quite a small emissivity of thermal radiation, an air mass flow rate per unit area of collector above 0.1 kg/m2·s, and maintaining the inlet fluid temperature close to that of the ambient fluid, will lead to improved thermal performance for both collectors. However, it is found that the mean gap between the cover and the absorbing plates, the emissivity of thermal radiation of the bottom plate, the solar insolation, and the inclination of the collectors have negligible effects on the efficiencies of both collectors.

Direct simulation of weak laminar plane fountains in a homogeneous fluid

Abstract The behaviour of weak laminar plane fountains that result from the injection of dense fluids upwards into a large container of homogeneous fluid of lower density is studied numerically by using a time-accurate finite volume code. The study is an extension of a previous numerical study on weak laminar axisymmetric fountains. Weak laminar plane fountains with both a uniform and a parabolic profile of the discharge velocity at the source have been investigated. The initial, temporary and final characteristic fountain heights and the times for the fountain front to reach these heights have been determined and scaled. At steady state, a height scale as well as a horizontal length scale for the fountain width are determined. The vertical distribution of the vertical velocity and temperature on the symmetry line are shown to scale with this height. The height and horizontal length scales have been used to scale the horizontal variation of both the horizontal and the vertical velocity in the fountain core at steady state.

An Experimental Study on the Heat Storage Performances of Polyalcohols NPG, TAM, PE, and AMPD and their Mixtures as Solid-Solid Phase-Change Materials for Solar Energy Applications

The heat storage performance of polyalcohols NPG (Neopentyl glycol), TAM (Tris(hydroxymethyl)aminomethane), PE (Pentaerythritol), and AMPD (Aminoglycol) and their mixtures as solid-solid phase-change materials for solar energy applications are experimentally investigated by the DSC (differential scanning calorimetry) technique. It is found that pure AMPD is not suitable for practical applications due to its wide ranges of charge and discharge temperatures and its high hygroscopic property. The specific sample preparation process has significant impact on the heat storage performance of the mixtures of polyalcohols. If the experimental samples are prepared with the proper processes, there will usually be only one peak presented in the DSC curves instead of two peaks, as reported by some studies. Due to the poor heat storage performance and high hygroscopic property of AMPD, the binary mixtures of AMPD/NPG and AMPD/TAM do not have practical application values. However, the binary mixtures of AMPD/PE with a high or low weight ratio of AMP still are found to have potential practical application values. The experimental results also show that a mixture of more than two polyalcohols do not have better performances than a binary mixture.

Unsteady natural convection boundary-layer flow along a vertical isothermal plate in a linearly stratified fluid with Pr > 1

Abstract In this study, the unsteady natural convection boundary-layer flow along an impulsively heated vertical isothermal plate immersed in a stably stratified semi-infinite ambient fluid is explored using scaling analysis and direct numerical simulation. Scaling relations are obtained for the thermal and velocity boundary layer thicknesses, the boundary layer velocity, the development time and the Nusselt number, in terms of the Rayleigh and Prandtl numbers and the stratification parameter. The scaling results are validated using the numerical simulations.