Accurate Heat Transfer Measurements Research Articles

An alternative post-processing of transient liquid crystal experiments using the wall temperature gradient time response

An alternative method for the fast evaluation of transient liquid crystal experiments is presented in this study. The calibration of liquid crystals has been considered essential to obtain accurate heat transfer measurements. Nevertheless, liquid crystal aging, illumination, and ex situ calibration effects remain a difficult and might influence the heat transfer level. Proposed here is to bypass liquid crystal calibration by utilizing the evolution of the wall temperature gradients. This necessitates the use of the detection times of the maximum red and green intensities of narrow bandwidth liquid crystals, e.g., less than 1 K, whose temperature span is on the order of 0.3 K. The calculation of the heat transfer coefficients can be thus feasible by differentiating the wall temperature from the solution of the semi-infinite body approach with respect to time. This approach is examined over different experiments with varying geometries and liquid crystal detection times. The in-advance possession of the calibration curves of the liquid crystals allows for the method’s cross-evaluation. The results indicate that the proposed method is in sufficiently good agreement with the values obtained using the calibrated liquid crystal signals. In particular, the average heat transfer coefficient of a surface is within 8% from the traditional method which is sufficient for industrial applications and preliminary thermal designs.

Heat transfer measurement techniques in microchannels for single and two-phase Taylor flow

Accurate measurement of heat transfer in experiments of single, two-phase liquid-liquid and gas-liquid Taylor flows without phase change in microchannels is a challenging task. In this paper we review experimental studies of these flows, and the methods of heat transfer measurement used, in order to gain insight into the effect of some critical dimensionless groups on the accuracy of the estimates of the heat transfer. It was found that experimental measurements of Nusselt number often disagree with theoretical predictions, particularly at low Reynolds numbers. We identify the major causes of error in experimental estimation of the Nusselt number, and propose solutions to decrease the error in experimental heat transfer measurement. Measurements obtained from a new microchannel experimental setup are reported upon, to demonstrate these sources of error.

Thermal and damage analysis of laser-induced decomposition within carbon/epoxy composite laminates

Fire-induced decomposition of composite materials used for aircraft structures involves complex and coupled multi-physics phenomena studied through conventional standard tests such as cone calorimeter, FAR25.856(b):2003 and ISO2685:1998(e), for instance. It is proposed to address this issue within controlled environment and accurate heat loading. Thermal response and damage evolution are investigated experimentally and numerically to analyse the anisotropic and heterogeneous behaviour of decomposing composite laminates subjected to laser heating and confined within the test chamber of the BLADE facility developed at ONERA. Accurate heat transfer measurements are correlated with numerical results from MoDeTheC solver and post-decomposition micrographic imaging in order to analyse the material thermo-chemical behaviour and identify the damage mechanisms.

Accurate heat transfer measurements using thermochromic liquid crystal. Part 1: Calibration and characteristics of crystals

Encapsulated thermochromic liquid crystal (TLC) can accurately measure surface temperature in a variety of heat transfer and fluid flow experiments. Narrow-band TLC, where the colour changes over a temperature range of ∼1 °C, can be used to determine surface temperature within an uncertainty of 0.1 °C. Wide-band TLC, typically active over 5–20 °C, allow the possibility of mapping surface temperature distributions. In part 1 of this two-part paper, an extensive set of calibrations for narrow-band and wide-band TLC is reported. This generic study provides insight into the importance and influence of the various factors governing the colour–temperature relationship. These governing effects include the variation in optical path, the spectrum of the illumination source, the lighting and viewing angles, the differences between cooling or heating cycles (hysteresis), the variation with the number of heating or cooling cycles (aging) and how this varies with TLC film thickness. Two narrow-band crystals are also specifically calibrated for application to experiments on a transparent disc rotating at high speed (∼5000 rpm). Part 2 of this paper describes how these accurately-calibrated crystals were used to measure the transient surface temperature on, and heat transfer to, a rotating disc.

Accurate heat transfer measurements using thermochromic liquid crystal. Part 2: Application to a rotating disc

Encapsulated thermochromic liquid crystal (TLC) can accurately measure surface temperature in a variety of heat transfer and fluid-flow experiments. In Part 1 of this two-part paper, two narrow-band liquid crystals were specifically calibrated for application to experiments on a disc rotating at high speed (∼5000 rpm). Part 2 describes how these crystals were used to measure the surface temperature on the disc in a transient experiment that models the flow of internal cooling air in a gas turbine. The TLC was viewed through the transparent polycarbonate disc using a digital video camera and strobe light synchronised to the disc frequency. The convective heat transfer coefficient, h, was subsequently calculated from the one-dimensional solution of Fourier’s conduction equation for a semi-infinite wall. The analysis accounted for the exponential rise in the air temperature driving the heat transfer, and for experimental uncertainties in the measured values of h. The paper focuses on the method used, and sample experimental results are provided to demonstrate the accuracy and potency of the technique.

Experiments and modeling of two-phase transient flow during CO2 pipeline depressurization

Abstract Accurate knowledge and models of transient and two-phase CO2 behaviour are important for safe and cost efficient design of CO2 pipel ines. A CO2 pipeline test rig has been successfully utilized for establishing the operation window and specifications of CO2 pipelines for verifying a theoretical two -phase transient flow model for depressurization and other transient operation of CO2 pipe lines. It is concluded that the model is experimentally verified within its validity rang. A second rig for accurate measurement of heat transfer out of CO2 pipelines has recently been started up.

Recommendations and guidelines for the performance of accurate heat transfer measurements in rapid forming processes

The published requirements for accurate measurement of heat transfer at the interface between two bodies have been reviewed. A strategy for reliable measurement has been established, based on the depth of the temperature sensors in the medium, on the inverse method parameters and on the time response of the sensors. Sources of both deterministic and stochastic errors have been investigated and a method to evaluate them has been proposed, with the help of a normalisation technique. The key normalisation variables are the duration of the heat input and the maximum heat flux density. An example of application of this technique in the field of high pressure die casting is demonstrated. The normalisation study, coupled with previous determination of the heat input duration, makes it possible to determine the optimum location for the sensors, along with an acceptable sampling rate and the thermocouples critical response-time (as well as eventual filter characteristics). Results from the gauge are used to assess the suitability of the initial design choices. In particular the unavoidable response time of the thermocouples is estimated by comparison with the normalised simulation.

Accurate Heat Transfer Measurements for Condensation on Horizontal, Integral-Fin Tubes

In most earlier experimental investigations of condensation on low-fin tubes, vapor-side heat transfer coefficients have been found from overall (vapor-to-coolant) measurements using either predetermined coolant-side correlations or “Wilson plot” methods. When the outside resistance dominates, or is a significant proportion of the overall resistance, these procedures can give satisfactory accuracy. However, for externally enhanced tubes, and particularly with high-conductivity fluids such as water, significant uncertainties may be present. In order to provide reliable, high-accuracy data, to assist in the development of theoretical models, tests have been conducted using specially constructed plain and finned tubes fitted with thermocouples to measure the tube wall temperature, and hence the vapor-side heat transfer coefficient, directly. The paper describes the technique for manufacturing the tubes and gives results of systematic heat transfer measurements covering the effects of fin height, thickness, and spacing, tube diameter, and vapor velocity. The tests were carried out with steam, ethylene glycol, and R-113, with vertical vapor downflow. The heat flux was measured using an accurately calibrated 10-junction thermopile and paying particular attention to coolant mixing and isothermal immersion of thermocouple junctions. Care was taken to avoid errors due to the presence in the vapor of noncondensing gas and the occurrence of dropwise condensation. Smooth, consistent, and repeatable results were obtained in all cases. The data are presented in easily accessible form and are compared with the results of previous investigations, where indirect methods were used to determine the vapor-side data, and with theory.