Energy Efficiency / Heat transfer fluids / Instrumentation

Effective Simulation Requires a Broader Scope of Process Heat Transfer Measurements

Accurate measurements under controlled conditions form the cornerstone of the development of methods for the successful design of heat transfer equipment.

By Kevin J. Farrell

Accurate measurements under controlled conditions form the cornerstone of the development of methods for the successful design of heat transfer equipment. For many years, overall measurements on industrial-sized heat exchangers were sufficient for model development and validation. More recently, it has become necessary to experimentally study portions of heat exchangers to validate the results of incremental design algorithms. Now, computational fluid dynamics tools allow engineers to compute the three-dimensional velocity, temperature, and pressure fields at very high spatial resolution—limited only by available computational resources. The successful application and validation of CFD tools require yet another significant refinement in the spatial resolution of experimental measurements. In addition to one-dimensional point measurements, distributions, i.e., two- and three-dimensional measurement, are required not only at the boundaries but across the flow. Notwithstanding the need for CFD validation, there are a myriad of important design issues worthy of more detailed experimental investigation: single- and two-phase flow, temperature, concentration, and species distributions in inlet and exit nozzles, across bundle entrance and exit planes, around distribution plates and impingement protection devices, and in inlet channels, turnaround headers, and ancillary piping.

Size and cost limitations make it difficult to study these phenomena comprehensively with conventional instrumentation—pressure transducers, pitot probes, and thermocouples. Many of these transducers are housed in probes that are intrusive—their presence in the flow intrinsically alters the value of the measurement. First introduced in the mid 1960s, laser-Doppler velocimetry (LDV) has been recognized as a very reliable, non-intrusive method of measuring velocity in complex flow fields such as chemically reacting flows, combustion, flames, and flows with radiation. The instrument can operate over wide velocity, density, temperature, and composition ranges. No calibration is required because the velocity is measured directly from light scattered by particles in the flow. While the active control volume is sufficiently small for point measurements, the focal point must be traversed accurately over the measurement domain, resulting in experiments of significant time duration.

Particle image velocimetry (PIV) overcomes the difficulties of controlling the experiment for a long time period by measuring the flow field very quickly. PIV uses a laser light sheet technique in which the light sheet is pulsed twice and images of fine seed particles lying in the light sheet are recorded on a video camera or a photograph. The displacement of the particle images is measured in the plane of the image and used to determine the displacement of the particles in the flow. The velocity associated with each interrogation region is the displacement divided by the time between the laser pulses. Two velocity components are measured, but a stereoscopic approach allows three velocity components to be recorded if necessary. Modern charge-coupled device (CCD) cameras and computing hardware result in near real-time velocity distributions. With sequences of velocity maps, statistical analyses can be used to yield spatial correlations.

Planar laser-induced fluorescence (PLIF) uses a procedure similar to PIV to measure instant whole-field scalars. PLIF links the fluorescent light emitted from molecules naturally present in the fluid with a given property such as temperature or concentration. In combination with PIV, transport properties and diffusion coefficients are measured. The system separates and samples the two signals simultaneously by optical filtering. The principal components necessary to carry out the PIV/PLIF measurements are a laser source, CCD cameras (an array of light detectors), optics, and a computer to run the data acquisition and processing software.

Because light is used in PIV/PLIF, the test fluid must be optically transparent, and the light must have a way to enter and leave the interrogation region—either through window(s) or fiber optics. PIV and PLIF have been used in hydrocarbon processes such as combustion, fuel injection, and fuel sprays. While some process fluids are transparent, many are not. Measurements in crude oil, for example, would not be possible currently; however, a new technique under development uses ultrasound instead of light to determine the velocity of the particles.

Clearly, optical measurement systems will greatly enhance the opportunities to obtain the high resolution data necessary to validate and use modern incremental design and CFD analysis tools effectively. Moreover, the ever-decreasing price of electronics makes these systems more affordable than ever.