Uniform and non-uniform inlet temperature of a vertical hot water jet injected into a rectangular tank
In most of real-world applications, such as the case of heat stores, inlet is not kept at a constant temperature but it may vary with time during charging process. In this paper, a vertical water jet injected into a rectangular storage tank is measured experimentally and simulated numerically. Two cases of study are considered; one is a hot water jet with uniform inlet temperature (UIT) injected into a cold water tank, and the other is a cold water jet with non-uniform inlet temperature (NUIT) injected into a hot water tank. Three different temperature differences and three different flow rates are studied for the hot water jet with UIT which is injected into a cold water tank. Also, three different initial temperatures with constant flow rate as well as three different flow rates with constant initial temperature are considered for the cold jet with NUIT which is injected into a hot water tank. Turbulence intensity at the inlet as well as Reynolds number for the NUIT cases are therefore functions of inlet temperature and time. Both experimental measurements and numerical calculations are carried out for the same measured flow and thermal conditions. The realizable k-ε model is used for modeling the turbulent flow. Numerical solutions are obtained for unsteady flow while pressure, velocity, temperature and turbulence distributions inside the water tank are analyzed. The simulated results are compared to the measured results, and they show a good agreement at low temperatures.
Numerical analysis of the transient turbulent flow field in a thermally stratified thermal storage water tank
Transient numerical analysis was carried out for the replacement process of cold fluid with hot fluid in a thermal storage water tank with a two-dimensional two-equation turbulence model (k−e model). The QUICKEST (QUICK with estimated streaming terms) method was two-dimensionalb) extended to solve the energy equations. The effect of the difference schemes (the QUICKEST scheme and the upwind scheme) to solve the energy equations was discussed on the transient flow and temperature fields. Comparison with existing experimental results showed that the present numerical approach can fairly predict the characteristics of thermal stratification in a thermal storage water tank.
Direct numerical simulation of buoyancy-driven turbulence in stably stratified fluid
We investigate the role of buoyancy force on the generation and decay of random motion in a homogeneously stratified fluid by means of direct numerical simulations (DNS) of the dynamic and thermodynamic equations. The simulations start from a fluid which is at rest but has appreciable temperature fluctuations. Therefore the flow initially evolves by extracting energy from the potential energy field. Three free parameters, the Reynolds number Re, the Prandtl number Pr and the stratification number St, characterize the flow. Among these numbers the stratification number, St = (lT0/T′0) (dTR/dz), is the most crucial one for the investigated problem. Here T′0 and lT0 are the initial r.m.s. temperature and the initial integral temperature lengthscale, respectively, and dTR/dz is the background stratification. St is a measure of the strength of background-temperature gradient compared to the initial mean fluctuating temperature gradient in the fluid. A critical stratification number of order one is found to separate an oscillating, non-turbulent flow from flow states which exhibit features of turbulence. When St > 1, the statistics reveal a nearly linear and strongly anisotropic flow as typical for gravity waves but the flow-field variables behave randomly. When St < 1, i.e. when the initial gradient of fluctuating temperature exceeds the gradient of its background value, the available potential energy is sufficient to create nonlinear motions which resemble turbulence in many aspects. The properties of such a flow are a transient state of enhanced stirring and mixing, enhanced rates of dissipation of temperature fluctuations, and a quick return to isotropy. The stratification number is an easily measurable parameter in field experiments in the ocean as well as in the atmosphere. Therefore St may be a useful indicator of whether a flow regime contains sufficient potential energy to create turbulence.