Transient flow analysis in a Roots blower: Experimental and numerical investigations

Abstract

It is widely acknowledged that rotary positive displacement machines exhibit highly unsteady flow fields that affect their performance. The presence of the operational clearances impacts this unsteady flow field and further affects the performance. However, the exact nature of these unsteady flow mechanisms remains largely unknown that necessitates both detailed experimental investigations and computational modelling. Thus, the present study employs both optical visualization and unsteady Reynolds-Averaged Navier Stokes (URANS) computational modelling methods while focussing on investigating the transient flow field inside a Roots blower, a general type of the rotary positive displacement machine. Straight lobes in a Roots blower provide convenient optical access to experimentally analyse internal flow and compare it with the predictions obtained by standard computational models. In the first part of this paper, this study covers the lowspeed experimental investigations using i) High-Speed Camera (HC), ii) the continuous High-Speed Particle Image Velocimetry (CPIV) and, iii) the instantaneous PIV (IPIV) obtained with a double pulse laser and a double shutter camera. Relative merits from these techniques are discussed with respect to the Roots blower unsteady flow mechanisms. In addition, computational analyses are performed using a combination of in-house and commercial modelling methods and the results are compared against the experiments. The results confirm the existence of highly three-dimensional and unsteady flow field where certain distinct flow mechanisms originating from the operational clearances impact the performance of the Roots blower. The study also highlights challenges of the experimental and computational methods used for evaluation of positive displacement machines that impact the accuracy of results.

Experimental and Numerical Investigation of Tip Leakage Flows in a Roots Blower

“…The phase-lock condition required every pair of images to be identical to each other at exact the same crank angle. However, the images change in angular position slightly from cycle to cycle because of fluctuations in the rotational speed [26]. We found that the images match each other after every 10 cycles.…”

“…The double-pulse laser (K) and double shutter camera (M) with the resolution of 2048 × 2048 pixels were used to take every pair of images at a short interval of 2-50 µs that was depended on the maximum particle velocity in the flow field. The equipment used for measurements was introduced in detail in the previously published paper by the authors [9,25,26]. For the measurement of gap flow field, following changes were made to the test set-up: Firstly, the microscope lens Model K2 (Infinity photo-optical, Centennial, CO, USA) was equipped on the double-shutter camera to magnify the tip gap (see Figure 3).…”

“…The adaptive PIV method is an automatic and adaptive cross-correlation method that iteratively adjusts the size and the shape of the individual interrogation areas (IA) in order to adapt to local seeding densities and flow gradient, further details of which could be found in [27]. And the detailed processing procedure for the images of gap flow is the same as that in a previous publication [26]. After processing a pair of images, the instantaneous velocity field were obtained.…”

“…However, as the screw machines have the helical rotors that limit optical access to the flow field, the quantitative velocity values in working chamber domains and gap domains cannot be obtained. Sun et al [9,25] and Singh et al [26] measured the velocity in the working chamber of a Roots blower with straight lobes using optical methods and compared the unsteady simulated main flow fields with the PIV test results. The three-dimensional and unsteady nature of the flow field was illustrated and the leakage flows through the three gaps were discussed.…”

“…The three-dimensional and unsteady nature of the flow field was illustrated and the leakage flows through the three gaps were discussed. In reference [26], the measured tip leakage flow was displayed without introducing the test rig for leakage flow and comparing with CFD results, so the validation of the leakage flow of the CFD model has not been fully achieved in the past and the leakage flow nature needs to be investigated further.…”