Takahiro Moroi

An Experimental Study on Opening Delay of a Reed Valve for Reciprocating Compressors

Automatic reed valves are widely used to control refrigerant gas flow in reciprocating compressors for automotive air conditioners. The oil film in the clearance between the reed and the valve seat causes a delay in opening of the valve. This opening delay of the discharge valve leads to over compression, which increases losses such as friction in sliding components and gas overheating. Therefore it is important to understand the behavior both of the oil film and the elastic reed deformation in order to reduce losses due to the delay. This study aims to develop an experimental setup that enables simultaneous visualization of the oil film rupture and measurement of the reed deformation, and to observe this behavior during the valve opening process. The gas-compression stroke is simulated by controlling compressed air with an electromagnetic valve. The oil film rupture is visually observed using a high speed camera through a special valve seat made of glass. The total deformation of the cantilever reed is identified by multipoint strain measurement with 12 strain gauges. The experiment finds that the opening process is divided into four stages. In the first stage, the reed remains stuck to the seat and deforms while the bore pressure increases. In the second stage, cavitation occurs in the oil film and the film starts to rupture. In the third stage, the oil film ruptures and the bore pressure starts to decrease. Finally, in the fourth stage, the reed is separated from the seat and the gas flows through the valve. Reducing the reed/seat contact area changes the reed deformation in the first stage, thereby increasing the reed/seat distance and realizing an earlier oil film rupture and a shorter delay.Copyright

Swirling Flow of Viscoelastic Fluid in a Cylindrical Casing

The swirling flow of viscoelastic fluid in a cylindrical casing is investigated experimentally, using aqueous solutions of 0.05–1.0 wt% polyacrylamide as the working fluid. The velocity measurements are made using laser Doppler anemometer. The aspect ratios H/R (H: axial length of cylindrical casing, R: radius of rotating disc) investigated are 2.0, 1.0 and 0.3. The Reynolds numbers Re0 based on the zero shear viscosity and the disc tip velocity are between 0.36 and 50. The velocity measurements are mainly conducted for the circumferential velocity component. The experimental velocity data are compared with the velocity profiles obtained by numerical simulations using Giesekus model and power-law model. It is revealed that at any aspect ratios tested the dimensionless circumferential velocity component Vθ ’ decreases with increasing Weissenberg number We. The Giesekus model could predict this retardation of circumferential velocity fairly well at small We, but the power-law model could not. The extent of the inverse flow region where the fluid rotates in the direction opposite to the rotating disc is clarified in detail.Copyright

Turbulence Characteristics of the Boundary Layer on a Rotating Cone in an Axial Uniform Flow.

The characteristics of turbulence in the boundary layer developed on a rotating cone in an axial uniform stream are investigated experimentally. Measurements of all the six components of the Reynolds stress and the triple products of the turbulent velocity fluctuations are made at various values of the surface-to-free stream velocity ratio λb for two different vertex angles, θ=15°and 30°. As λb increases, the destabilizing effect of the centrifugal force becomes stronger, leading to the formation of regular large-scale vortices. The structural change of turbulence caused by these vortices affects the distribution of the statistical turbulence quantities significantly. In the outer region, the direction of the shear stress vector does not align with that of the velocity gradient vector. The ratio of the integral lengh scale to the Taylor microscale, L/λ, increases as the destabilizing centrifugal effect increases.

THE STRUCTURE OF TURBULENCE IN THE BOUNDARY LAYER ON A SPINNING CONE IN AN AXIAL UNIFORM FLOW

The structure of turbulence in the boundary layer on a 15° cone spinning in an axial uniform flow was investigated experimentally. Flow visualization studies revealed the formation of large–scale spiral eddies at large values of the surface-to-free stream velocity ratio. A triple-sensor probe was used to obtain the three instantaneous velocity components. The spectra and the probability density distributions of the fluctuating velocity components indicated a strong influence of large-scale eddies on the turbulence structure. To study the effects of large-scale eddies on the primary turbulent shear stress component the four-quadrant method was applied to the v- and w- components, where v and w represent the fluctuating velocity components in the azimuthal and wall-normal directions. The mean burst periods were evaluated for respective regions of large-scale outflow and inflow.