Dr Michael J Pekris

Brush seals offer a superior sealing effectiveness compared to labyrinth seals. However, widespread use of brush seals is constrained by deleterious behaviors such as pressure-stiffening and hysteresis. For the latter, the bristles bend during the shaft incursion process and do not fully recover during the shaft retraction process. An opening gap is created, which increases seal leakage unless the pressure load drops to a certain level. In the present work, analytical and numerical models based on a single bristle are proposed to capture the seal's response to shaft displacement with and without pressure loading. The models are validated using static stiffness tests at an unpressurized condition from literature. The main results show that modeling of the backing ring friction is essential to capture the bristle hang-up behavior. Shaft friction dominates at unpressurized conditions, while backing ring friction dominates at high pressure loading. An expression for shaft hang-up displacement has been derived. A sensitivity study shows that seals with shallow lay angle, short bristle length, and large bristle diameter are less prone to hang-up problems. The models developed in the present framework have been shown to qualitatively capture the pressure stiffening, hysteresis, bristle hang-up, and shaft rotation effects.

Transcritical carbon dioxide waste heat recovery systems and the construction of scroll expanders have recently been hot topics. The flank clearance, located between the orbiting and fixed scroll, has a vital impact on the scroll expander performance. This paper estimates the effect of the flank clearance on the expander’s thermodynamic performance (first-law efficiency) based on computational fluid dynamics (CFD) simulations. The manufacturing cost of different flank clearances is also considered to enhance the feasibility of the machinery design. The computational cost for different flank clearance cases is significantly reduced with a surrogate-assisted multi-objective optimisation algorithm (SAMOA), which also supports modelling the trade-off relationship between manufacturing cost and machinery efficiency. The results indicated that an increasing flank clearance negatively affects the first-law thermal efficiency. The efficiency decreased from 87.41% to 44.83% moving from 20 to 200 μm flank clearances. The SAMOA successfully reduced the computational cost of the dynamic mesh CFD model from 90 h to 15 s with 0.6% discrepancy. The final Pareto solutions presented a clear trade-off relationship between the first-law efficiency and manufacturing cost and promised a diversity of optimum solutions. The “knee points” for the relationship were 25, 55, and 127 μm, which provided flexible clearance choices based on the importance of either machinery efficiency or manufacturing cost.

Recent developments in the field of renewable energy have led to a renewed interest in low-grade heat (< 500 K). The low-grade heat is widely wasted by the lack of efficient heat recovery technologies. It is also limited by the system size, which defines as the micro to small-scale (< 50 kW). Although ORC based unit has been implemented in this field, the CO2 based waste heat recovery units can be more capable in the size construction. The performance of the expander plays a vital role in the system's efficiency. Thus, the current paper provides thermodynamic and CFD analysis of a scroll expander regarding a micro-scale T-CO2 recovery system (< 10 kW) with a 400 K low-grade heat source. In the current CFD model, all the fluid domains were constructed by structural mesh. It also successfully integrated with the thermodynamic table to simulate two-phase T-CO2. This model can be the first scroll expander model for T-CO2 power system and gap the bridge of utilising the scroll machinery in this field. The CFD methodology was successfully validated by the new-built testing platform and previous data. The energy performance of T-CO2 and ORC (R123) based scroll expanders are compared by isentropic and exergy efficiency. The results showed that isentropic and exergy efficiencies of T-CO2 were 7% and 14% higher than the R123. It also identified higher irreversibilities of T-CO2 by the exergy of the working fluids. The pressure and temperature distributions identified the over-expansion and reversed flow characteristics, and the pressure imbalance of the initial expansion chambers denoted the reversed flow.

Brush seals can be used in the secondary air systems of gas and steam turbines to reduce parasitic leakages and engine specific fuel consumption. Application of these seal types in highly pressurised and highly swirling environments is limited due to the inherent risk of aerodynamic instability and seal failure. This paper considers coupled 3D CFD and structural modelling of brush seals, and applies the technique to investigate the effects of inlet swirl on the bristle pack. The model applies aerodynamic forces generated by CFD to a bristle pack model that includes interaction between bristles. Iteration between CFD and structural models is used to ensure consistency between the fluid and structural solutions. Inlet swirl is shown to increase bristle circumferential aerodynamic forces. At a critical value of inlet swirl, the aerodynamic force on the upstream bristles is sufficient to cause circumferential slip of the upstream bristle row, despite axial compression of the bristle pack under its operating differential pressure. 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, and in fact further lowers the critical swirl velocity where bristle slip occurs. This is associated with highly swirling flow impinging on the bristle tips. 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.

The secondary air system of a modern gas or steam turbine is configured to satisfy a number of requirements, such as to purge cavities and maintain a sufficient flow of cooling air to key engine components, for a minimum penalty on engine cycle efficiency and specific fuel consumption. Advanced sealing technologies, such as brush seals and leaf seals, are designed to maintain pressures in cavities adjacent to rotating shafts. They offer significant reductions in secondary air parasitic leakage flows over the legacy sealing technology, the labyrinth seal. The leaf seal comprises a series of stacked sheet elements which are inclined relative to the radial direction, offering increased axial rigidity, reduced radial stiffness, and good leakage performance. Investigations into leaf seal mechanical and flow performance have been conducted by previous researchers. However, limited understanding of the thermal behavior of contacting leaf seals under sustained shaft contact has led to the development of an analytical model in this study, which can be used to predict the power split between the leaf and rotor from predicted temperature rises during operation. This enables the effects of seal and rotor thermal growth and, therefore, implications on seal endurance and rotor mechanical integrity to be quantified. Consideration is given to the heat transfer coefficient in the leaf pack. A dimensional analysis of the leaf seal problem using the method of extended dimensions is presented, yielding the expected form of the relationship between seal frictional power generation, leakage mass flow rate, and rotor temperature rise. An analytical model is derived which is in agreement. Using the derived leaf temperature distribution formula, the theoretical leaf tip temperature rise and temperature distributions are computed over a range of mass flow rates and frictional heat values. Experimental data were collected in high-speed tests of a leaf seal prototype using the Engine Seal Test Facility at Oxford University. These data were used to populate the analytical model and collapsed well to confirm the expected linear relationship. In this form, the thermal characteristic can be used with predictions of mass flow rate and frictional power generated to estimate the leaf tip and rotor temperature rise in engine operation.

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.

When subject to highly swirling inlet flow, the bristles on the upstream face of a brush seal in gas turbine engines tend to slip circumferentially, which may lead to aeromechanical instability and seal failure. In this article, a new design of the front plate of brush seal, which mitigates this effect, is presented. Angled ribs on the upstream side of the front plate are used to reduce the swirl of the flow impacting on the bristle pack. The effects of the rib geometry, including angle of inclination and height-to-spacing ratio, are investigated using computational fluid dynamics, and a bulk porous medium model of the bristle pack, on a simple seal geometry. Results show that the ribs can effectively regulate the flow upstream of the bristle pack, reducing the swirl and channeling flow radially inward to the sealing section, resulting in decreased circumferential forces on the bristles. Ribs inclined at 20° to the radial direction and with height-to-spacing ratio of 0.4 were selected as the most effective of those investigated for the seal geometry under study. A model of an aeroengine preswirled cooling air chamber was created to give insight into the inlet swirl boundary conditions that a preswirl seal brush seal could be subjected to at a range of leakage flow rates and inlet swirl velocities. The new design and upstream roughness feature substantially reduced inlet swirl velocity incident on the bristle pack. The findings in this work could have a significant impact on brush seal design and, in particular, mitigate a significant operational risk of swirl-induced instability in high-pressure, high-speed shaft seal locations.

In recent decades, the carbon dioxide cycles, including supercritical carbon dioxide cycle, transcritical carbon dioxide Rankine cycle and refrigeration cycle, have been proven effective due to the high efficiency and compact structure, and received increasing interests. The performance of the expander in the power cycles, particularly in micro-scale applications, is one of the essential components that determine the cycle performance and still remains a significant challenge. This paper presents a critical overview of micro-scale (

As conventional aircraft designs approach their limits in terms of efficiency and emissions, a drastic change to the architecture of conventional platforms is required if the environmental targets of the next several decades are to be met. Boundary Layer Ingestion is one of industry’s most promising answers to the challenges of the future, identifying a potential step-change in performance in more integrated propulsion and airframe systems. This paper investigates the behaviour of a boundary layer ingesting solution of a closely embedded wing-electric ducted fan design, with focus on the implications of the aerodynamic coupling on the individual performance of both the aerodynamic and propulsive elements as well as on the assessment of the reliability of a low order panel code method. Wind tunnel testing was undertaken to understand the flow physics at different combinations of airframe and propulsor operating conditions; in addition, part of the data used for the experimental validation of a panel method model for predicting the upstream inlet flow conditions. It was found that there were clear local and extended upstream effects of the propulsor on the performance of the aerodynamic surface, resulting from the different combinations of suction strength and nacelle blockage. Similar trends were observed in the numerical code predictions, and identified limitations of the methodology in defining the experimental boundary conditions of the propulsor to be imposed in CFD. The study of the response of the propulsor to varying inlet boundary conditions, created by varying wing angles of attack was also carried out, however, small changes in flow velocity combined with measurement errors of the current system, prevented any solid conclusions being drawn about the impact of distorted inlet flow on propulsor performance.