Time-Resolved Heat Transfer Measurements Research Articles

Measuring firebrand heat flux with a thin-skin calorimeter

While the impact of wildland-urban interface fires is growing, firebrand exposure is a significant but not well understood contributor to fire spread. The ignition threat of firebrand exposures can be characterized by measuring the heat transfer of glowing firebrands to a surface. The current study presents a novel method for conducting time-resolved heat transfer measurements from individual firebrands across a range of flow conditions. Experiments are conducted with individual glowing firebrands generated from birch discs and placed on a copper thin skin calorimeter of the same diameter, which is embedded in the substrate. The net heat flux from the firebrand to the thin skin calorimeter is obtained from the thermal energy storage in the thin skin calorimeter, plus heat conduction losses to the substrate. Values of peak net heat flux, total heating, duration of heating are reported under different flow conditions from 0.05 m/s to 1.6 m/s. The average peak net heat flux for the disc-shaped birch firebrands is 45 kW/m2, and does not change significantly with flow condition. However, there is an increase in the total heating, duration of heating, and total mass consumed as the flow velocity increases.

Improved Methodologies for Time-Resolved Heat Transfer Measurements, Demonstrated on an Unshrouded Transonic Turbine Casing

The high pressure (HP) rotor tip and over-tip casing are often life-limiting features in the turbine stages of current gas turbine engines. This is due to the high thermal load and high temperature cycling at both low and high frequencies. In the last few years, there have been numerous studies of turbine tip heat transfer. Comparatively fewer studies have considered the over-tip casing heat transfer. This is in part, no doubt, due to the more onerous test facility requirements to validate computational simulations. Because the casing potential field is dominated by the passing rotor, to perform representative over-tip measurements a rotating experiment is an essential requirement. This paper details the measurements taken on the Oxford turbine research facility (OTRF), an engine-scale rotating turbine facility which replicates engine-representative conditions of Mach number, Reynolds number, and gas-to-wall temperature ratio. High density arrays of miniature thin-film heat-flux gauges were used with a spatial resolution of 0.8 mm and temporal resolution of ∼120 kHz. The small size of the gauges, the high frequency response, and the improved processing methods allowed very detailed measurements of the heat transfer in this region. Time-resolved measurements of TAW and Nu are presented for the casing region (−30% to +125% CAX) and compared to other results in the literature. The results provide an almost unique data set for calibrating computational fluid dynamics (CFD) tools for heat transfer prediction in this highly unsteady environment dominated by the rotor over-tip flow.

Bubble impingement and the mechanisms of heat transfer

Heat transfer augmentation resulting from the effects of two phase flow can play a significant role in convective cooling applications. To date, the interaction between a rising gas bubble impinging on a heated horizontal surface has received limited attention. Available research has focused on bubble dynamics and the associated heat transfer has not been reported. To address this, this study investigates the effect of a single bubble impinging on a heated downward facing horizontal surface. Local and time resolved heat transfer measurements have been performed for a 30mm orifice to surface distance, with a bubble injection orifice of 1mm in diameter. Synchronized high-speed photography and infrared thermography have been utilized to investigate the path of the bubble and the associated heat transfer.

Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor—Part I: Sensor and Benchmarks

A novel heat flux sensor was tested that allows for time-resolved heat flux measurements in internal ribbed channels related to the study of passages in gas turbine blades. The working principle of the atomic layer thermopile (ALTP) sensor is based on a thermoelectric field created by a temperature gradient over an yttrium-barium-copper-oxide (YBCO) crystal (the transverse Seebeck effect). The sensors very fast frequency response allows for highly time-resolved heat flux measurements up to the 1MHz range. This paper explains the design and working principle of the sensor, as well as the benchmarking of the sensor for several flow conditions. For internal cooling passages, this novel sensor allows for highly accurate, time-resolved measurements of heat transfer coefficients, leading to a greater understanding of the influence of fluctuations in temperature fields.

Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor—Part II: Heat Transfer Results

Measurements using a novel heat flux sensor were performed in an internal ribbed channel representing the internal cooling passages of a gas turbine blade. These measurements allowed for the characterization of heat transfer turbulence levels and unsteadiness not previously available for internal cooling channels. In the study of heat transfer, often the fluctuations can be equally as important as the mean values for understanding the heat loads in a system. In this study, comparisons are made between the time-averaged values obtained using this sensor and detailed surface measurements using the transient thermal liquid crystal technique. The time-averaged heat flux sensor and transient TLC results showed very good agreement, validating both methods. Time-resolved measurements were also corroborated with hot film measurements at the wall at the location of the sensor to better clarify the influence of unsteadiness in the velocity field at the wall on fluctuations in the heat flux. These measurements resulted in turbulence intensities of the velocity and heat flux of 20%. The velocity and heat flux integral length scales were about 60% and 35% of the channel width, respectively, resulting in a turbulent Prandtl number of 1.7 at the wall.

The influence of non-equilibrium dissociation on the flow produced by shock impingement on a blunt body

We describe an investigation of the effects of non-equilibrium thermochemistry on the interaction between a weak oblique shock and the strong bow shock formed by a blunt body in hypersonic flow. This type of shock-on-shock interaction, also known as an Edney type IV interaction, causes locally intense enhancement of the surface heat transfer rate. A supersonic jet is formed by the nonlinear interaction that occurs between the two shock waves and elevated heat transfer rates and surface pressures are produced by the impingement of the supersonic jet on the body. The current paper is motivated by previous studies suggesting that real gas effects would significantly increase the severity of the phenomenon. Experiments are described in which a free-piston shock tunnel is used to produce shock interaction flows with significant gas dissociation. Surprisingly, the data that are obtained show no significant stagnation enthalpy dependence of the ratio of the peak heat transfer rates with and without shock interaction, in contrast to existing belief. The geometry investigated is the nominally two-dimensional flow about a cylinder with coplanar impinging shock wave. Holographic interferometry is used to visualize the flow field and to quantify increases in the stagnation density caused by shock interaction. Time-resolved heat transfer measurements are obtained from surface junction thermocouples about the model forebody. An improved model is developed to elucidate the finite-rate thermochemical processes occurring in the interaction region. It is shown that severe heat transfer intensification is a result of a jet shock structure that minimizes the entropy rise of the supersonic jet fluid whereas strong thermochemical effects are promoted by conditions that maximize the entropy rise (and hence temperature). This dichotomy underlies the smaller than anticipated influence of real gas effects on the heat transfer intensification. The model accurately predicts the measured heat transfer rates. Improved understanding of the influence of real gas effects on the shock interaction phenomenon reduces a significant element of risk in the design of hypersonic vehicles. The peak heat transfer rate for the Edney type IV interaction is shown to be well-correlated, in the weak impinging shock regime, by an expression of the form [equation] for use in practical design calculations.