I. Owen

Numerical and experimental study of flow and heat transfer around a tube in cross-flow at low Reynolds number

Abstract A numerical and experimental study of the laminar flow and heat transfer characteristics of a cylinder in cross-flow is presented. The computational technique used is a stream function-vorticity formulation of the laminar flow steady state incompressible Navier–Stokes and energy equations and uses a Gauss–Seidel over-relaxation technique to obtain stream function and temperature distributions. Calculations are presented for an isothermally heated single tube in a duct with different blockage ratios. The variation of local Nusselt number, pressure and also isotherm and streamline contours are predicted with Reynolds number of 120 and 390. For the Reynolds number of 390, the local Nusselt number distributions are shown to be similar to those obtained through measurement of the local heat flux from the surface of a tube using a micro-foil heat flow sensor.

Integrating CFD and piloted simulation to quantify ship-helicopter operating limits

This paper describes a study which has been concerned with numerical predictions of the airwakes resulting from two simplified ship geometries: the internationally agreed Simple Frigate Shape, SFS1, and its successor, SFS2. Extensive steady-state simulations have been carried out for a wide range of wind conditions using Fluent, a commercially available Computational Fluid Dynamics (CFD) code. The CFD predictions have been partially validated against wind tunnel data produced by the National Research Council of Canada (NRC) and have shown good agreement. The resulting airwake velocity components have been exported from Fluent, interpolated onto suitable grids and attached to the FLIGHTLAB flight-simulation environment as look-up tables; piloted flight trials were then carried out using the Liverpool full-motion simulator. The pilot workload and helicopter control margins resulting from a range of wind-over-deck conditions have been used to develop the Ship-Helicopter Operating Limits (SHOL) for a Lynx-like helicopter and the SFS2. The workload was compared to the pilot’s experiences on a similar aircraft and a Type 23 Frigate and the simulated SHOL compared with SHOLs derived from sea trials. The results are very encouraging and open up further the long awaited prospect of such simulations being used in the future to reduce at-sea trials, and to provide a safe environment for pilot training.

Heterogeneous flashing in water drops

This paper is concerned with the vaporization of superheated water drops. When calculating the heat and mass transfer from drops in this condition, it is not known to what temperatures the drops will superheat before they react through explosive flashing. An experimental study has been carried out in which water drops with diameters between 1 and 3 mm have been suspended on a thermocouple junction in a saturated steam environment within a vessel at pressures up to 9 bar abs. By rapidly depressurizing the vessel at different rates so that the drop superheated, it was possible to measure the temperature of the drop and to observe its reaction. For superheat temperatures up to 5°C the drops experienced only quiescent surface evaporation. For superheat temperatures between 5 and − 18° C, the drops reacted through internal boiling but did not disintegrate. For temperatures above about 18°C the drops reacted by flashing explosively.

Impingement heat transfer in an under-expanded axisymmetric air jet

Abstract This paper is concerned with the heat transfer that occurs when an underexpanded jet impinges onto a heated surface. The heat transfer in the impingement zone is extremely high and when the surface interferes with the expansion of the jet, the radial distribution of the heat transfer coefficient becomes more complex. If the jet impinges upon the surface before the core of the jet has decayed, there is no longer a maximum stagnation heat transfer coefficient on the geometric axis of the jet, instead a stagnation ‘ring’ is formed with a radius of about one nozzle diameter. Experimental results are presented for nozzle pressure ratios up to 5.08, and nozzle-to-surface spacings of 3, 6 and 10 nozzle diameters. In addition to the measured data, a clear outcome of the work is that the usual method of representing Nusselt number as a function of Reynolds number is inadequate in compressible flows where the dimensional analysis shows that the nozzle Mach number, or pressure ratio, should also be included.

THE FLOW OF THIN LIQUID FILMS AROUND CORNERS

Abstract Situations arise where it is required to strip a moving liquid film from a boundary wall. The need to sample wet steam isokinetically is one such situation. Equally it is sometimes desirable for a film not to separate from a boundary wall as in, for example, liquid separators. A theoretical analysis is developed to examine the radial stress distribution within a uniformly thin liquid film flowing around a sharp bend of fixed radius. The results of the analysis are discussed in the light of experimental observations. The controlling parameters in the film flow are identified and are evaluated for a given situation.

Pressure loss and heat transfer through heat sinks produced by Selective Laser Melting

Heat removal from electronic packages is often assisted with the use of heat sinks whose heat transfer surfaces come in a variety of forms such as cylindrical pins, flat fins, and corrugated sheet. These conventional designs are manufactured by traditional methods such as forging, machining, casting, stamping and bending, or a combination of processes. This article introduces a novel manufacturing technique, selective laser melting (SLM), and demonstrates its ability to fabricate new designs of heat sink that have not previously been considered, primarily due to their geometric complexity. Three novel finned structures have been manufactured and their thermal and fluid flow characteristics have been determined experimentally. The three heat sinks demonstrate selective laser meltings ability to produce fine detail and consist of a staggered elliptical array, an elliptical array where the pins are angled in a direction perpendicular to the flow, and a densely packed diamond array. The novel heat sink designs were compared to a cylindrical pin array manufactured using the SLM process as well as with pin fin data from the literature. The heat sinks produced by the SLM method have been shown to have superior performance to that of the conventional heat sinks. Although the angled elliptical fins transmit similar amounts of heat to the cylindrical pin fins across the range of air flow rates considered, they incur a lower pressure loss. The densely packed diamond array not only transfers 60% more heat than the cylindrical array, but does so with a lower pressure drop across it.

Diffusing a homogenized two-phase flow

Abstract This paper describes the results of an experimental study in which the pressure recovery from a homogenized two-phase flow in a conical diffuser was measured. The flow was an air/water mixture with volumetric void fractions up to 35%. Although the pressure recovery was reduced because of the two-phase mixture, the use of a diffuser is still beneficial. For example, whereas a 7° diffuser operating in a single-phase flow achieves a pressure recovery of about 85%, the same diffuser operating in a flow with a 20% void fraction has a pressure recovery of about 70%; this compares with about 20% through a sudden expansion. It has been found that the optimum angle of the diffuser in two-phase flow is the same as that in single-phase flow, i.e. 7°. The pressure recovery coefficient has been defined using the homogeneous density and the velocity of the mixture at the inlet to the diffuser. An expression is proposed for predicting the pressure recovery coefficient of a diffuser operating in two-phase flow.

Ship-Helicopter Operating Limits Prediction Using Piloted Flight Simulation and Time-Accurate Airwakes

This paper gives an overview of the ship―helicopter dynamic interface simulation facility at the University of Liverpool, with an emphasis on recent improvements made through the inclusion of unsteady computational fluid dynamics (CFD) ship airwake data. A FLIGHTLAB model of an SH-60B Seahawk helicopter has been flown in a full motion base simulator to the deck of a Type 23 frigate and a Wave class auxiliary oiler, under the influence of unsteady airwakes derived from CFD. Pilot workload ratings have been obtained for the deck landing task, using both the Bedford workload rating scale and the deck interface pilot effort scale, from which fully simulated ship―helicopter operating limits have been derived. Analysis of pilot ratings, comments, and control inputs has also enabled both subjective and objective assessments of workloads at various wind-over-deck conditions, highlighting the dominant aerodynamic airwake features which contribute to the difficulty of the landing task. Having access to the underlying CFD data allows the aircraft handling qualities and pilot workload to be correlated with the aerodynamic characteristics of the airwake and identification of the geometric features of the ship that cause them.

A practical solution to the problem of noise and vibration in a pressure-reducing valve

Abstract The mechanical vibration that is occasionally found in gas pressure reducing valves can be eliminated by careful design of the valve plug and seat. A pressure-reducing valve was found to be excessively noisy, producing a sound pressure level of 117 dB when throttling air at an inlet-to-outlet pressure ratio of 15; as a result the valve suffered wear and vibration damage. By changing the design of the original flat plug and seat, the problem was significantly reduced. A 60° conical plug and seat produced a noise reduction of 12 dB (a factor of 4 in actual sound pressure level), the mechanical vibration was eliminated, and the flow capacity was increased by about 25%.

Accelerated surface erosion by cavitating particulate-laden flows

Abstract This short paper describes the results of a series of tests that were carried out to assess the erosion suffered by a fluidic valve controlling sand-laden water. Two valves were arranged in series so that each handled the same flows, but, because of the pressures in the system, the water in the second valve was cavitating. With only cavitation erosion the surface of the valve was merely discoloured. With particulate erosion in the non-cavitating flow there was some material removal by abrasion. With both cavitation and sand particles in the flow there was very considerable material removal, much more than would be expected from summing the two individual effects.