Numerical investigation on the compressor for gas-entraining diffuser EGR

Exhaust Gas Recirculation (EGR) is an effective way to reduce nitrogen oxide (NO x) emissions, and the EGR application increases the engine backpressure to some extent. In this paper, a new EGR method named gas-entraining diffuser EGR was proposed to reduce pumping loss. It introduces the exhaust gas into the compressor diffuser inlet where the static pressure is the lowest without blades fouled by exhaust gas. As a result, lower pressure at the turbine upstream can achieve EGR. Then, a newly designed induced structure not only introduces exhaust gas into the compressor diffuser but also reduces the energy loss caused by EGR application. Furthermore, the performance of compressor with different induced angles of the induced structure was investigated using simulation method. Results showed that the compressor’s adiabatic efficiency was the best when the induced angle was 20°. Regarding the induced angle of 20°, the adiabatic efficiency drop of compressor was in the range of 0.8%–12%. Approximately 10% of the adiabatic efficiency drop was caused by the induced structure, the other was mainly from the flow loss and mixing loss in diffuser system. The induced structure mainly affected the static pressure difference between induced structure inlet and impeller outlet ([Formula: see text]). When the impeller mass flow was 0.23 kg/s, [Formula: see text] was 11.21, 13.95, 15.59, 17.18 kPa respectively with corresponding induced angles of 20°, 30°, 40°, 50°. The primary energy loss leading to the adiabatic efficiency drop of compressor with induced structure occurred in diffuser system. It was caused by the mixing process of induced gas and impeller exit gas, and the enhanced effect from the shroud side’s impeller jet-wake and volute tongue.

Investigation of Rotating Stall Phenomenon and Optimization in Mixed-Flow Waterjet Propulsion Device

The rotating stall is a kind of flow phenomenon in the impeller harming the navigation speed of vessels propelled by a waterjet propulsion device when the waterjet propulsion device operates at the small flow rate conditions. The numerical simulated hydraulic performance was compared with experimental results so that the reliability of the CFD method was verified. The grooves are proposed before the inlet of an impeller to suppress the rotating stall. The orthogonal experiment is designed to seek the appropriate values of the parameters such as the length, width, depth, and number of the groove. The results show that the width of groove has the greatest influence on the performance of the device, followed by the number, the depth, and the length. The width, number, depth, and length of the selected groove scheme are 3.10 × 10−2D, 72, 3.10 × 10−2D, and 7.75 × 10−2D, respectively. At the rotating stall conditions, the selected groove scheme is numerically calculated. In contradistinction to the original scheme, it is found that the groove can improve the flow in the impeller and enhance the head of valley point condition, but the efficiency drops due to the increasing local hydraulic loss near the groove. When the groove is installed, the positive slop region on the flow rate-head curve disappears related to the static pressure difference on the blade, especially depending on the static pressure on the suction side. The flow pattern on the span surface of the vortex core is smooth, and then, the head increases. On the contrary, the head decreases. The outcome is beneficial to improve the performance of the waterjet propulsion device and enrich the rotating stall theory.

The Effect of Inlet Swirl on Brush Seal Bristle Deflections and Stability

This paper considers three-dimensional (3D) computational fluid dynamics (CFD) and structural modeling of brush seals, and investigates the effects of inlet swirl on the bristle pack. The model couples aerodynamic forces generated by CFD to a structural model that includes interaction between bristles. At a critical value of inlet swirl, aerodynamic forces cause circumferential slip of the upstream bristle row. In practice, this may lead to instability of the bristle pack and is consistent with anecdotal reports of seal behavior. The critical swirl velocity was reduced when the downstream pressure level was raised, keeping the same upstream total to downstream static pressure difference. This is caused by the increased dynamic head associated with the inlet swirl. Inclusion of a front plate in the seal design does not offer the intended protection to the bristle pack in highly swirling environments. This is associated with highly swirling flow impinging on the bristle tips. Fitting of roughness elements on the upstream face of the front plate could improve stability by reducing swirl of the flow impacting on the bristles. Increasing the bristle diameter and bristle stiffness does not necessarily prevent slip at higher inlet swirl velocities, but reduces the magnitude of slip of the upstream bristles.

Numerical Analysis of Contraction Geometry Effects on Cavitation Choking in a Piping System

There is a contraction portion in the water supply pipe line system, and cavitation may occur in the contraction when the flow velocity is increased. Such a situation occurs widely in the throat of the fluid machineries and in the vicinity of the valve body of the valve. In operation of the valve, it is well known that a phenomenon occurs in which the flow rate does not increase even if the static pressure difference upstream and downstream of the valve is increased due to the growth of cavitation in the contraction, which is well known as choking . It is not clear what phenomena occurs when cavitation surge occurs in the pipe system in the situation where choking is occurring in the contraction. In this study, cavitation CFD was performed on pipes those have three different geometry contractions. It was revealed that choking occurred when cavitation occurred in any shape. Also, in the case with the sharp contraction part and the sudden expansion, the flow fluctuation at the upstream of the contraction is much weaker than that at the downstream, but in the contraction with the bent part where the centrifugal force acts on the flow, the flow fluctuation at the upstream was found to be strong.

Reducing Compressor Shroud Leakage Flows by Raising the Stator Hub Line: Low Speed Tests

The paper describes experimental investigations of an alternative shrouded stator concept in a 2.5 stage low speed compressor. The idea of this new concept is to raise the stator hub line by a small amount, thus decelerating the flow upstream of the shroud cavity due to the into wind step and raising the static pressure. Downstream of the cavity the out of wind step changes the streamline curvature thus lowering the static pressure locally. As a result, the static pressure difference across the shroud is lower and the shroud flow is reduced. Tests were done at three seal gap heights under stator 1, both with a “neutral” (in–line) hub and a six percent “bump shroud”, i.e. the hub is raised by six percent annulus height. Performance measurements show the impact of the “bump shroud” geometry on the overall behavior of the compressor, i.e. efficiency and pressure ratio and the variation of these quantities with varying seal gap height. While the efficiency and pressure ratio of the compressor inevitably reduce with increasing seal gap height, the sensitivity of both is reduced by using “bump shrouds”. At small seal gap heights the “bump shroud” design behaves similarly to the neutral one, while at the design seal gap height it is superior. Thus, both the efficiency and the pressure ratio are less sensitive against seal gap height variations if the compressor is equipped with a raised hub line — leading to a more robust product. A similar behavior is seen at near stall conditions. The analysis of five hole probe measurements reveals the reason for the improved efficiency. The stator 1 losses were significantly reduced by the introduction of the “bump shroud”. This is mostly due to the reduced amount of shroud flow and the subsequent reduction of hub cross–flow in the stator. A comparison of losses with and without the raised hub line show not only a reduction of the losses near the hub, but also adjacent to the suction side of the stator due to reduced migration of hub boundary layer fluid onto the vane.

The Effect of Inlet Swirl on Brush Seal Bristle Deflections and Stability

This paper considers 3D CFD and structural modelling of brush seals, and investigates the effects of inlet swirl on the bristle pack. The model couples aerodynamic forces generated by CFD to a structural model that includes interaction between bristles. At a critical value of inlet swirl, aerodynamic forces cause circumferential slip of the upstream bristle row. In practice this may lead to instability of the bristle pack and is consistent with anecdotal reports of seal behavior. The critical swirl velocity was reduced when the downstream pressure level was raised, keeping the same upstream total to downstream static pressure difference. This is caused by the increased dynamic head associated with the inlet swirl. Inclusion of a front plate in the seal design does not offer the intended protection to the bristle pack in highly swirling environments. This is associated with highly swirling flow impinging on the bristle tips. Fitting of roughness elements on the upstream face of the front plate could improve stability by reducing swirl of the flow impacting on the bristles. Increasing the bristle diameter and bristle stiffness does not necessarily prevent slip at higher inlet swirl velocities, but reduces the magnitude of slip of the upstream bristles.

Effects on the Ventilation of a Two-Storey Building under Different Thermal Conditions

In this paper it is shown how the air flow rate of decentralized ventilation devices can be affected by a staircase of a two-storey building under different thermal conditions. Since these devices need local fans for supplying the requested volume flow, pressure loads have a significant impact on the delivered volume flow rates. Regarding this, the study comprises two analyses: First, a CFD-model is developed to simulate the ventilation air flow through a simplified staircase. By varying parameters for rooms’ temperature and ventilation direction, the hydrostatic pressure in the staircase is evaluated. The simulations – characterized by high Archimedes numbers – are successfully validated with findings from preliminary work. In a second part, the pressure conditions inside the staircase are referred to outside conditions. Consequently, a static pressure difference at the ventilation device on each storey can be observed. We found that the deliverable volume air flow rate can decrease up to 10 % from the nominal flow rate due to temperature differences between the storeys and outside. Therefore, heat recovery and ventilation effectiveness may also be impaired.

Experimental Study on Discharge Coefficients of Windward Window in Buildings with Wind-Driven Cross Ventilation

Focus on the prediction of flow rates in buildings under natural ventilation, the investigation conducted a series of model rooms with cross ventilation. The impact of window-wall ratios, windows configurations as well as corresponding flow rates was investigated. The object of this investigation is to analyze characteristics of windward window opening discharge coefficient by measuring static pressure difference and the flow rate through windows. The conclusion are as follows: For large openings, the discharge coefficient of windward window opening increases as the window-wall ratio grows up; With windward window-wall ratio of 44.4% and 11.1%, the discharge coefficient of windward openings is almost irrelevant to flow rate and less affected by leeward window area; However, with windward window-wall ratio of 2.78%, the discharge coefficient increases slightly as flow rate rises, and the larger the area of leeward opening is, the smaller the discharge coefficient of windward opening becomes.