The use of the interaction of convex wall jets for ventilation with variable air flow

The most energy efficient ventilation and air-conditioning is variable air flow (VAV) depending on the needs of a room. To avoid broken air circulation by gravitational forces, the most of air diffusers should change geometrical shape and sizes using additionall automation of them. In contrast, high stability of a scheme of air exchange organization with air supply over a working zone by convex wall jets that interact with each other under conditions of variable air flow, is confirmed. This scheme is useful in cases where it is impossible to supply air directly to the working zone. Simulation of the air exchange organization in an exhibition hall of International Exhibition Centre in Kyiv with ventilation at a variable air volume (VAV) in the entire possible range of performance control has been performed. The floor area is 5258 m2, the height is 19 m. The outdoor air-flow at design conditions (100 % load) is 21.667 m3/s (78000 m3/h). The minimum load corresponds to the absence of solar radiation and only some people in the room. The minimum air-flow is 25 % of the design one. The proposal air scheme is single-zonal using 24 diffusers PES-D-8-10/15-0,9 4 m above the floor and air removal from the upper zone. The air distributor have a diameter of a cylindrical surface and an inlet branch pipe of 8 dm (800 mm). There are 10 rows of nozzles at an angle π/12 (15 °) to the horizon on each distributor. The total area of the air outlet on them is equal to 0.9 of the cross-sectional area of the inlet pipes. Due to forces of the vacuum holding of jets on the wall surfaces, the influence of gravitational forces is significantly reduced. This avoids the automation of air distribution devices to stabilize the scheme of air circulation in the room by gravitational forces. It is enough to install valves with actuators on branches of a network of air ducts. Thus, the economic benefit of the system is confirmed both at the stage of installing and during operation.

Study on the Collapse Process of Cavitation Bubbles Including Heat Transfer by Lattice Boltzmann Method

In this study, an improved double distribution function based on the lattice Boltzmann method (LBM) is applied to simulate the evolution of non-isothermal cavitation. The density field and the velocity field are solved by pseudo-potential LBM with multiple relaxation time (MRT), while the temperature field is solved by thermal LBM-MRT. First, the proposed LBM model is verified by the Rayleigh–Plesset equation and D2 (the square of the droplet diameter) law for droplet evaporation. The results show that the simulation by the LBM model is identical to the corresponding analytical solution. Then, the proposed LBM model is applied to study the cavitation bubble growth and collapse in three typical boundaries, namely, an infinite domain, a straight wall and a convex wall. For the case of an infinite domain, the proposed model successfully reproduces the process from the expansion to compression of the cavitation bubble, and an obvious temperature gradient exists at the surface of the bubble. When the bubble collapses near a straight wall, there is no second collapse if the distance between the wall and the bubble is relatively long, and the temperature inside the bubble increases as the distance increases. When the bubble is close to the convex wall, the lower edge of the bubble evolves into a sharp corner during the shrinkage stage. Overall, the present study shows that this improved LBM model can accurately predict the cavitation bubble collapse including heat transfer. Moreover, the interaction between density and temperature fields is included in the LBM model for the first time.

Air distribution in convex wall jets for ventilation with a constant air flow

The scheme of air exchange organization using air supply above a working zone by convex wall jets that interact with each other has been substantiated. This scheme is advisable in cases where it is impossible to supply air directly to the working zone. It provides optimal microclimate parameters with minimal recirculation of polluted air from the upper zone. Simulation of the air exchange organization in an exhibition hall in International Exhibition Centre in Kyiv with ventilation at a constant air volume (CAV) has been performed. The floor area is 5258 m2, the height is 19 m, the minimum outdoor air flow is 21.667 m3/s (78000 m3/h). The current design scheme of air exchange organization is zonal. General air exchange is 43.3333 m3/s (156000 m3/h). Recirculation is accepted 50 %. The air flow supplied in the upper and middle zones is, respectively, 22.5 m3/s (81000 m3/h) and 20.833 m3/s (75000 m3/h). Inlet air has temperature 291.65 K (18.5 °C). It is supplied downward by twisted jets. There are 65 Trox VDL-AHLD-E3/800/0/0/0/RAL 9010 air diffusers with a diameter of 800 mm. The proposed scheme is single-zonal using 24 diffusers PES-D-8-10/15-0,9 4 m above the floor and air removal from the upper zone. This scheme allows halving the air exchange to the minimum outdoor air without recirculation. The air temperature should be decreased by 3.3 K to 288.35 K (15.2 °С). The number of air-conditioners is decreased twice. The calculated consumption of cold decreased by 65.58 W/m2 or 29 %, the calculated consumption of heat for the second heating – by 7.17 W/m2 or 18 %. Saving of capital investments in prices of February 2020 is 792.16 UAH/m2 or 55 %, and decrease of operating costs for the cooling period is 6.61 UAH/m2 or 15 %. Thus, the system is economically beneficial from the beginning of its installation. In the future, its operation will be simulated in a mode with a variable flow rate.

Nonlinear behaviour of convergent Richtmyer–Meshkov instability

A novel shock tube is designed to investigate the nonlinear feature of convergent Richtmyer–Meshkov instability on a single-mode interface formed by a soap film technique. The shock tube employs a concave–oblique–convex wall profile which first transforms a planar shock into a cylindrical arc, then gradually strengthens the cylindrical shock along the oblique wall, and finally converts it back into a planar one. Therefore, the new facility can realize analysis on compressibility and nonlinearity of convergent Richtmyer–Meshkov instability by eliminating the interface deceleration and reshock. Five sinusoidal $\text{air}{-}\text{SF}_{6}$ interfaces with different amplitudes and wavelengths are considered. For all cases, the perturbation amplitude experiences a linear growth much longer than that in the planar geometry. A compressible linear model is derived by considering a constant uniform fluid compression, which shows a slight difference to the incompressible theory. However, both the linear models overestimate the perturbation growth from a very early stage due to the presence of strong nonlinearity. The nonlinear model of Wang et al. (Phys. Plasmas, vol. 22, 2015, 082702) is demonstrated to predict well the amplitude growth up to a normalized time of 1.0. The prolongation of the linear increment is mainly ascribed to the counteraction between the promotion by geometric convergence and the suppression by nonlinearity. Growths of the first three harmonics, obtained by a Fourier analysis of the interface contour, provide a first thorough validation of the nonlinear theory.