Flow Test Bench Research Articles

Experimental Study on Flow Transition in Dual Bell Nozzles

The dual bell nozzle is a concept of altitude adaptive nozzles. The flow adapts to the altitude by separation at the wall inflection at low altitude, and full flowing at high altitude. To understand the phenomenology of the flow by the transition from sea level to high altitude mode, a series of tests have been made at the cold flow test bench P6.2 at DLR Lampoldshausen. Three nozzles with different geometries have been tested. Two of them were successively shortened and driven unter the same conditions for each extension length. This study yields the influence of the geometric parameters of the basis and the extension on the transition conditions. Furthermore a transition prediction is given.

Flow transition in dual bell nozzles

The dual bell nozzle is a concept of altitude adaptive nozzles. The flow adapts to the altitude by separation at the wall inflection at low altitude, and full flowing at high altitude. To understand the phenomenology of the flow by the transition from sea-level mode to high altitude mode, a series of tests have been made at the cold flow test bench P6.2 at DLR Lampoldshausen. A dual bell nozzle have been successively shorten and driven in the same conditions for each length. Hence it was possible to observe the flow in the vicinity of the wall inflection using Schlieren optics. Furthermore this study yields the influence of the length ratio between basis and extension on the transition conditions.

Time to surge concept and surge control for acceleration performance

Surge is a dangerous instability that can occur in compressors. It is avoided using a valve that reduces the compressor pressure. The control of this valve is important for the compressor safety but it also has a direct influence on the acceleration performance. Compressor surge control is investigated by first studying the surge phenomenon in detail. Experimental data from a dynamic compressor flow test bench and surge cycles measured on an engine is used to tune and validate a model capable of describing surge. A concept named time to surge is introduced and a sensitivity analysis is performed to isolate the important characteristics that influence surge transients in an engine. It is pointed out that the controller clearly benefits from a feed-forward term due to the small time frames associated with the transition to surge. In the next step this knowledge is used in the design of a novel surge controller. This surge controller is then compared to two other controllers and it is shown that it avoids surge and improves the acceleration performance by delivering both higher engine torque and turbo shaft speed after a gear change.

Design and optimisation of a multi-powder feed system for the HVOF deposition process

Currently no specific method exists for the deposition of High Velocity Oxy-Fuel (HVOF) thermal spray functionally graded coatings. This paper investigates the design and optimisation of a multi-powder HVOF thermal spray device in order to deposit aluminium/tool-steel functionally graded coatings. A multi-powder feed device concept was developed. The concept was based on a stand alone two powder chamber device which integrates with common hopper systems to allow the mixing of two powders during thermal spray deposition. This concept was verified by simulation the design of this device for multi-powder flow using Finite Element Analysis (FEA) to arrive at the optimum dual feed design. The FEA model predicted the mixing and flow of two powders of various ratios' of mass flow rate and velocity based on an optimum designed shape and pressure ratios' of nitrogen gas in the chamber to pick-up shaft of 2.25:1. This yielded the best results in terms of carrying the powders from the mixing zone into the nitrogen gas flow path, inside the pick-up shaft and on towards the HVOF gun. Post finite element analysis the device was manufactured for the utilisation within the HVOF process. Optimisation tests of the device included; powder flow bench tests and HVOF thermal spraying of graded deposits. The results revealed a calibration graph for the two powders in question and the compositional variation across the deposit during functionally graded deposition. The composition of the graded deposits were close to that anticipated hence this showed the suitability of the newly designed multi-powder deposition system in mixing two powders for the purpose of producing HVOF graded coatings.

PORT DESIGN OPTIMIZATION USING CFD ANALYSIS

Shape of ports that are part of an engine cylinder head is vital to engine performance and emissions. The advance of CFD (Computation Fluid Dynamics) analysis technology helps designers run the simulation to improve the port design and to provide the better model for a flow bench test. This paper presents the automation of design optimization process integrating CAD modeling, mesh generation and CFD simulation.

Active Combustion Instability Control With Spinning Valve Actuator

Active combustion control has been accomplished in many laboratory and real-world combustion systems by fuel modulation as the control input. The modulation is commonly achieved using reciprocating flow control devices. These demonstrations have been successful because the instabilities have been at relatively low frequencies (∼200 Hz) or the scale of demonstration has been small enough to require very small levels of modulation. A number of real-world instabilities in gas turbine engines involve higher frequencies (200–500 Hz) and attenuation requires the modulation of large fractions of the engine fuel flow rate (hundreds of pounds per hour). A spinning drum valve was built to modulate fuel for these applications. Tests showed that this device provided more than 30% flow modulation up to 800 Hz for liquid fuel flows of greater than 400 lbm/hr. This paper describes the performance of the valve in flow bench tests, open-loop forcing, and closed-loop instability control tests. The closed-loop tests were done on a single-nozzle combustor rig which exhibited a limit-cycling instability at a frequency of ∼280 Hz with an amplitude of ∼7 psi. It also encounters an instability at 575 Hz under a different set up of the rig, though active control on that instability has not been investigated so far. The test results show that the spinning valve could be effectively used for active instability control, though the control algorithms need to be developed which will deal with or account for actuator phase drift/error.

Experimental study on axial stratification process and its effects (I)-Stratification in Engine-

This paper is the first of several companion papers, which investigate axial stratification process and its effects in an SI engine. The axial stratification is very sophisticate phenomenon, which results from combination of fuel injection, port and in-cylinder flow and mixing. Because of the inherent unsteady condition in the reciprocating engine, it is impossible to understand the mechanism through the analytical method. In this paper, the ports were characterized by swirl and tumble number in steady flow bench test. After this, lean misfire limit of the engines, which had different port characteristic, were investigated as a function of swirl ratio and injection timing for confirming the existence of stratification. In addition, gas fuel was used for verifying whether this phenomenon depends on bulk air motion of cylinder or on evaporation of fuel. High-speed gas sampling and analysis was also performed to estimate stratification charging effect. The results show that the AFR at the spark plug and LML are very closely related and the AFR is the results of bulk air motion.

Modeling and Experimental Determination of the Axisymmetric Flow in a Diesel Engine Cylinder

Internal air motion in a Detroit Diesel 6V-92 two-stroke engine cylinder was determined by an idealized axisymmetric finite difference model under steady flow conditions. This model was used to compute axial and tangential velocities with entrance angles of 25 and 40 deg. Laser Doppler anemometer measurements were made in one plane for comparison under steady flow bench test conditions using actual engine hardware. Calculations and measurements showed that the swirl induced by the angled inlet flow was responsible for the nonuniform axial velocity profiles. These profiles exhibited large peaks in the midradius region (between cylinder center and wall). Increasing the inlet angle from 25 to 40 deg sharpened the axial velocity peak and shifted it toward the cylinder wall. The measurements indicated nearly isotropic turbulence across the measurement plane. A better understanding of the relationship between design and internal flow is expected to lead to improved combustion, scavenging tradeoffs.