Christopher Long

Review of Temperature Measurement

A variety of techniques are available enabling both invasive measurement, where the monitoring device is installed in the medium of interest, and noninvasive measurement where the monitoring system observes the medium of interest remotely. In this article we review the general techniques available, as well as specific instruments for particular applications. The issues of measurement criteria including accuracy, thermal disturbance and calibration are described. Based on the relative merits of different techniques, a guide for their selection is provided.

Heat Flux Measurement Techniques

Abstract Heat flux measurement is used in the field of fluid mechanics and heat transfer to quantify the transfer of heat within systems. Several techniques are in common use, including: differential temperature sensors such as thermopile, layered resistance temperature devices or thermocouples and Gardon gauges; calorimetric methods involving a heat balance analysis and transient monitoring of a representative temperature, using, for example, thin-film temperature sensors or temperature sensitive liquid crystals; energy supply or removal methods using, for example, a heater to generate a thermal balance; and, finally, by measurement of mass transfer which can be linked to heat transfer using the analogy between the two. No one method is suitable to all applications because of the differing considerations of accuracy, sensitivity, size, cost and robustness. Recent developments including the widespread availability and application of thin-film deposition techniques for metals and ceramics, allied with advances in microtechnology, have expanded the range of devices available for heat flux measurement. This paper reviews the various types of heat flux sensors available, as well as unique designs for specific applications. Critical to the use of a heat flux measurement technique is accurate calibration. Use of unmatched materials disturbs the local heat flux and also the local convective boundary layer, producing a potential error that must be compensated for. The various techniques in common use for calibration are described. A guide to the appropriate selection of a heat flux measurement technique is provided according to the demands of response, sensitivity, temperature of operation, heat flux intensity, manufacturing constraints, commercial availability, cost, thermal disturbance and acceleration capability for vibrating, rotating and reciprocating applications.

A Review of Forced Convective Heat Transfer in Stationary and Rotating Annuli

The study of heat transfer by forced convection in annular passages is of interest across the range of process and aeronautical industries, for example from annular heat exchangers to the various configurations of annuli found in turbomachinery. The aim of this paper is to review relevant experimental, numerical and analytical research of heat transfer in both stationary and rotating annuli, with an emphasis on presenting useful information for designers. The geometries considered are the stationary annulus with superposed axial throughflow and the rotating annulus with rotation of either the inner or outer cylinder (both with and without throughflow). The work presented covers laminar and turbulent flows as well as flow regimes where transition occurs or vortex flows are present.

A combined experimental and theoretical study of flow and pressure distributions in a brush seal

A relatively simple theory is presented that can be used to model the flow and pressure distribution in a brush seal matrix. The model assumes laminar, compressible, isothermal flow and requires knowledge of an empirical constant: the seal porosity value. Measurements of the mass flow rate together with radial and axial distributions of pressure were taken on a nonrotating experimental rig. These were obtained using a 122 mm bore brush seal with 0.25 mm radial interference. The experimental data are used to estimate the seal porosity. Measurements of the pressure distributions along the backing ring and under the bristle tips and discussed. Predicted mass flows are compared with those actually measured and there is reasonable agreement considering the limitations of the model.

Numerical computation of laminar flow in a heated rotating cavity with an axial throughflow of air

A heated rotating cavity with an axial throughflow of cooling air is used as a model for the flow in the cylindrical cavities between adjacent discs of a high‐pressure gas‐turbine compressor. In an engine the flow is expected to be turbulent, the limitations of this laminar study are fully realised but it is considered an essential step to understand the fundamental nature of the flow. The three‐dimensional, time‐dependent governing equations are solved using a code based on the finite volume technique and a multigrid algorithm. The computed flow structure shows that flow enters the cavity in one or more radial arms and then forms regions of cyclonic and anticyclonic circulation. This basic flow structure is consistent with existing experimental evidence obtained from flow visualization. The flow structure also undergoes cyclic changes with time. For example, a single radial arm, and pair of recirculation regions can commute to two radial arms and two pairs of recirculation regions and then revert back to...

Experimental investigation and mathematical modeling of clearance brush seals

In this paper, an experimental program and a CFD based mathematical model using a brush seal at two bristle to rotor clearances (0.27 mm and 0. 75 mm) are presented. The experimental program examined the radial pressure distributions along the backing ring, the axial pressure distribution along the rotor, and the mass flow through the seal through a range of pressure ratios while exhausting to atmosphere. The results from this experimental program have been used to further calibrate a CFD-based model. This model treats the bristle pack as an axisymmetric, anisotropic porous region, and is calibrated by the definition of nonlinear resistance coefficients in three orthogonal directions. The CFD analysis calculates the aerodynamic forces on the bristles, which are subsequently used in a separate program to estimate the bristle movements, stresses, and bristle and rotor loads. The analysis shows that a brush seal with a build clearance produces a very different flow field within the bristle pack to one with an interference, and the need to understand the bulk movements of the bristles. These are shown to be affected by the level of friction between the bristles and the backing ring, which has an important effect on the bristles wear and seal leakage characteristics.

Shroud Heat Transfer Measurements from a Rotating Cavity with an Axial Throughflow of Air

The paper discusses measurements of heat transfer obtained from the inside surface of the peripheral shroud. The experiments were carried out on a rotating cavity, comprising two 0.985-m-dia disks, separated by an axial gap of 0.065 m and bounded at the circumference by a carbon fiber shroud. Tests were conducted with a heated shroud and either unheated or heated disks. When heated, the disks had the same temperature level and surface temperature distribution. Two different temperature distributions were tested; the surface temperature either increased, or decreased with radius. The effects of disk, shroud, and air temperature levels were also studied

Disk heat transfer in a rotating cavity with an axial throughflow of cooling air

Abstract A heated, rotating cavity with an axial throughflow of air is used as an experimental model for a pair of gas turbine, high-pressure compressor disks. Tests were carried out on a single rotating cavity comprising two disks of radius b = 0.4845m, bounded at the circumference by a carbon fiber shroud. Experiments were conducted with and without a heated shroud and for the range of parameters: 0.03 ≤ βΔT ≤ 0.3; 2 × 103 ≤ Rez ≤ 16 × 104 and 2 × 105 ≤ Reφ ≤ 5 × 106; for cavity gap ratios G = 0.13 and 0.36, and a constant value of inlet radius ratio of a/b = 0.1. Measurements were also made of the air temperature inside the cavity by three thermocouple probes. The heat transfer from the disks was measured using thermopile flux meters. The measurements of cavity air temperature and cavity heat transfer were used to estimate the fraction of the central throughflow entering the cavity. This shows only a slight dependence on the gap ratio. For Ro 10, this decreases to around 10 percent. Two mechanisms appear to operate in influencing the heat transfer. Firstly, heating of the air inside the cavity destablizes it and convection occurs under the action of rotationally induced buoyancy forces. Secondly, the central throughflow encourages mixing with the cavity air, which can either affect the heat transfer directly (as, for example, at the inner radii of the disks) or indirectly through the action of vortex breakdown. For the smaller gap ratio cavity, rotational effects become increasingly important toward the outer radius across a wide range of Rossby numbers. For the wider gap ratio cavity, this is restricted to a narrower range of Ro. In the region 4 ≤ Ro ≤ 5, the gap ratio plays a crucial role in affecting the disk heat transfer. At smaller values of Ro, where a significant fraction of the throughflow enters the cavity, the disk heat transfer rate does not appear to be affected by the gap ratio. At larger values, increasing the gap ratio also increases the heat transfer.

Measurement and Computation of Heat Transfer in High-Pressure Compressor Drum Geometries With Axial Throughflow

This paper makes comparisons between CFD computations and experimental measurements of heat transfer for the axial throughflow of cooling air in a high-pressure compressor spool rig and a plane cavity rig. The heat transfer measurements are produced using fluxmeters and by the conduction solution method from surface temperature measurements. Numerical predictions are made by solving the Navier-Stokes equations in a full three-dimensional, time-dependent form using the finite-volume method. Convergence is accelerated using a multi grid algorithm and turbulence modeled using a simple mixing length formulation. Notwithstanding systematic differences between the measurements and the computations, the level of agreement can be regarded as promising in view of the acknowledged uncertainties in the experimental data, the limitations of the turbulence model and, perhaps more importantly, the modest grid densities used for the computations.

HEAT TRANSFER IN HIGH-PRESSURE COMPRESSOR GAS TURBINE INTERNAL AIR SYSTEMS: A ROTATING DISC-CONE CAVITY WITH AXIAL THROUGHFLOW

This article reports on heat transfer measurements made on a rotating test rig representing the internal disc-cone cavity of a gas turbine high-pressure (H.P.) compressor stack. Tests were carried out for a range of flow rates and rotational speeds at engine representative nondimensional conditions. The rig also had a central drive shaft, which could rotate in the same direction as the discs, contrarotate relative to the discs, or remain static. Measurements of heat transfer were obtained from a conduction solution method using measured surface temperatures as boundary conditions. Results from the outer surface of the cone are in reasonable agreement with theoretical predictions for the heat transfer from a free cone in turbulent flow. The heat transfer measurements from the inner surface of the cone reveal two regimes of heat transfer: one governed by rotation, the other by action of the throughflow. In the rotationally dominated regime, the heat transfer from the inner surface of the cone is higher for a co-rotating shaft than for either a static or contra-rotating shaft. In the throughflow-dominated regime the heat transfer shows little consistent dependence on the direction of shaft rotation. Tests carried out at different values of surface-to-fluid temperature difference add support to the hypothesis that in the rotationally dominated regime the heat transfer occurs through a process of free convection, where the buoyancy force is induced by rotation. The heat transfer from the disc is significantly lower than that from the inner surface of the cone and more or less insensitive to the sense of shaft rotation. The disc average Nusselt numbers show similar behavior to those from the inner surface of the cone and suggest that the disc heat transfer too is governed either by rotationally induced buoyancy or by the axial throughflow.This article reports on heat transfer measurements made on a rotating test rig representing the internal disc-cone cavity of a gas turbine high-pressure (H.P.) compressor stack. Tests were carried out for a range of flow rates and rotational speeds at engine representative nondimensional conditions. The rig also had a central drive shaft, which could rotate in the same direction as the discs, contrarotate relative to the discs, or remain static. Measurements of heat transfer were obtained from a conduction solution method using measured surface temperatures as boundary conditions. Results from the outer surface of the cone are in reasonable agreement with theoretical predictions for the heat transfer from a free cone in turbulent flow. The heat transfer measurements from the inner surface of the cone reveal two regimes of heat transfer: one governed by rotation, the other by action of the throughflow. In the rotationally dominated regime, the heat transfer from the inner surface of the cone is higher for ...