Influence of Thermocouple Thermal Inertia in Impingement Heat Transfer Experiments Using Transient Techniques

A typical transient heat transfer experiment for an impingement cooling configuration requires usually a temperature step in the flow. A moving isotherm of the liquid crystal surface coating is then monitored using a video camera. However, for small impingement configurations, when only a small number of holes is used, the massflow rates are relatively low, leading to very small plenum velocities. Therefore, a step change in the power settings does not cause a temperature step in the mainstream flow. Many researchers approximated the real hot gas temperature evolution by a number of ideal temperature steps (Duhamel’s superposition principle). However, thermocouple acquisition measurements are influenced by the size of the thermocouple due to thermal inertia and therefore direct evaluation of the heat transfer coefficient with these data may be in error. This paper suggests that the hot gas temperature which drives the transient experiment and is measured in the plenum can be corrected according to the time constant of the thermocouples. Several experiments were carried out in order to evaluate the time response of fine thermocouples with exposed junction. For the impingement configuration, a single row of five inline impingement holes is used at in a narrow passage configuration over a range of Reynolds numbers (15000-55000). The liquid crystal signal is evaluated with three different hot gas temperature approaches: (1) Perfect temperature step, (2) Duhamel’s principle in the acquired temperature data and (3) Correction of hot gas temperature for thermal inertia prior to Duhamel’s principle. The experimental data are analyzed by means of various post-processing procedures and aim to clarify and quantify the effect of thermocouple thermal inertia on the final results.

Hole Staggering Effect on the Cooling Performance of Narrow Impingement Channels

Peter Ott, Alexandros Terzis, Guillaume Wagner

This study examines experimentally the cooling performance of integrally cast impingement cooling channels which provide increased heat transfer area compared to traditional impingement configurations. For the evaluation of the heat transfer coefficient, the transient liquid crystal method was used. Full surface heat transfer coefficient distributions on the target plate and the side walls of the channel have been measured by recording the temperature history of liquid crystals using a frame grabber. Several impingement cooling geometries have been tested composing a test matrix of nine different geometrical configurations. The experimental data are analyzed by means of various post-processing procedures and aim to clarify and quantify the effect of hole staggering on the overall cooling performance, a variable which has been little addressed in the open literature. The experiments were carried out in a low speed wind tunnel over a wide range of Reynolds numbers between 15’000 and 100’000. The results indicated similarities with convectional multi-jet impingement cooling systems as well as a noticeable effect of the cooling hole pattern. Finally, an error propagation analysis of the experimental uncertainties was performed providing information for the significance of scatter on repeated experiments.

2012

Thermocouple Thermal Inertia Effects on Impingement Heat Transfer Experiments Using the Transient Liquid Crystal Technique

Peter Ott, Alexandros Terzis, Bernhard Weigand

The transient liquid crystal technique is widely used for impingement heat transfer experiments. Additionally, due to the difficulty of producing pure temperature steps in the flow, many authors assumed the fluid temperature evolution as a series of step changes using Duhamel’s superposition theorem. However, for small impingement configurations where the jets are fed from the same plenum chamber, and hence flow velocities are relatively small, thermal inertia of commercial thermocouples causes a delay, lagging from the real plenum temperature history. This paper investigates thermal inertia characteristics of thermocouples and their effect on the calculation of impingement heat transfer coefficient. Several thermocouples with exposed junction and different wire diameter were considered over a range of plenum flow conditions typically found in impingement heat transfer experiments. The effect of thermocouple time constant on the evaluation of the heat transfer rate was investigated in a narrow channel consisting of five inline impingement jets. The results indicated a significant effect of thermocouple response on the stagnation point region heat transfer, while lower local heat transfer rates are negligibly affected as liquid crystal signals appear later in time and the driving gas temperature history has a smaller influence on the evaluated data.

Institute of Physics2012

Experimental and Numerical Investigation of a Fully Confined Impingement Round Jet

Oriana Caggese, Gabriel Charles Gnaegi, Gwenael Hannema, Peter Ott, Alexandros Terzis

The heat transfer characteristics of a fully confined impingement jet are experimentally and numerically evaluated. Full surface heat transfer coefficient distributions are obtained for the target and impingement plate of the model using the transient liquid crystal technique and a commercial CFD solver. The confined box consists of a single round jet impinging over a flat surface at relatively low jet-to-target plate distances, varied between 0.5 and 1.5 jet diameters. The impingement geometry is blocked from the three sides, and therefore, the air of the jet is forced to exit the model in a single direction resulting in a fully confined configuration. Experiments were carried out over a range of Reynolds varying between 16,500 and 41,800. The experimental data is compared to the numerical simulations aiming to quantify the degree of accuracy to which the heat transfer rates can be predicted.

Pergamon-Elsevier Science Ltd2013