By Dr Mustapha Chaker, Mee Industries and Dr Paul Kippax, Malvern Instruments
The inlet fogging of gas turbines for power augmentation has seen increasing application over the past decade, but little information about the physics and engineering of the process is available. Inlet fogging can be used to augment the power of a gas turbine by up to 20%. To achieve this level of augmentation, however, a number of important parameters must be understood, including fog droplet size, droplet kinetics and duct behavior of the droplets.
A major problem faced by gas turbine users considering the utilization of inlet fogging is that different fog nozzle manufacturers and suppliers present data in very different formats and under different operating conditions. With so many parameters to measure and so many variables which can affect those measurements, the creation of a measurement protocol is important.
The American of Society of Mechanical Engineers (ASME) is in the process of developing a measurement protocol. Adherence to this protocol should provide consistent inter-nozzle performance rankings for tests carried out in fogger nozzle supplier, turbine engine manufactures, and turbine system purchaser laboratories. A draft of the protocol is currently being prepared by ASME. Currently there is no date when the standard will be available.
In drafting the protocol, ASME consulted both nozzle system vendors and nozzle system users. When completed the document will describe the experimental configurations that should be used for droplet size and velocity measurement using the techniques of laser diffraction and PDA (Phase Doppler Anemometry). It will also include the parameters that should be measured and reported. Once issued, the standard will enable gas turbine manufacturers and nozzle manufacturers to assess the performance of fogging nozzle systems in a reproducible way. This will then allow fogging systems from different manufacturers to be reliably compared.
For the creation of this protocol, researchers at Mee Industries have developed a set of guidelines for inlet fogging measurement1 using laser diffraction technology. This is the technology recommended in the ASME draft protocol.2
Mee’s work aims to define the instrumentation to be used for inlet fogging analysis as well as the locations where measurements are to be made; the airflow velocities under which the measurements should be made; the averaging approach and representative diameters to be used; and the final report format to be used.
For droplet size measurements, use of a laser-diffraction particle sizer, such as the Spraytec system from Malvern Instruments, is recommended. A laser diffraction particle size analyzer measures the intensity of light scattered from droplets passing through a laser beam. Because the angle at which droplets scatter light is inversely proportional to their size, the droplet particle size distribution can be directly calculated from the spray’s scattering pattern. A patented multiple scattering algorithm incorporated into the Spraytec allows the accurate measurement of both dense and diffuse spray plumes.
Using this technique the time to take measurements is very fast, acquiring size distributions at a rate of up to 2,500 Hz. This allows for the measurement of short-duration sprays that reveal fine temporal fluctuations in the atomizer output. Continuous measurements over longer periods of time are also possible but at lower acquisition rates. This enables users to understand the stability of different nozzle systems and to directly track the effect of changing atomizer conditions on the delivered particle size.
It has been found that measurements taken at different locations in a gas turbine or wind tunnel can yield very different results.1 Measurements of droplet diameters at distances progressively further downstream from the fogging nozzles results in progressively larger diameters, Figure 1. While this increase, far from the nozzle, may be due to evaporation of smallest droplets, it is due essentially to coalescence close to the nozzle tip.
The coalescence importance increases with the increase of the flow rate of the nozzle and the decrease in plume cone angle close to the nozzle orifice. To take into account the coalescence effect, droplet size should be taken at 3 inches and 10 inches from the nozzle tip. The droplet size at the center of the plume is smaller than the size at the edge, and, depending on which type of nozzle is used, the distribution can vary.
The velocity of the air also has a major impact on the particle size distribution, Figure 2. It is therefore important that all measurements are taken at the same air velocity. Measurements should be made at flow velocities of 2.5 m/sec (8.3 ft/sec) and 12.7 m/s (41.7 ft/sec), which are reasonable velocities for typical locations of inlet fogging manifolds in gas turbine ducts close to the inlet filter housing and after the silencer. Research has shown that increasing the airflow velocity above 5.1 m/sec does not reduce droplet size significantly.1
To achieve reproducible and reliable test data, all tests should be conducted on a minimum of five randomly selected nozzles within error bars of 10%. Measurements should be taken across the spray plume. It is essential that enough readings be taken to cover the entire cross section of the plume. In addition, to ensure measurement repeatability, the measurement duration for each section measured should be a least one minute.
Analysis of results
For gas turbine operations, the two most important droplet diameter results are Dv90 and D32. Dv90 is the representative diameter where 90% of the total volume of the liquid sprayed is made up of droplets
with diameters smaller than or equal to the stated value. This representative diameter is commonly used to characterize gas turbine inlet air fogging nozzles. However, there is concern that the ingestion of large droplets by the axial flow compressor might cause blade erosion or blade coating wear.
D32 is the Sauter Mean Diameter and is calculated using the concept of the volume-to-surface-area ratio. It is equal to the sum of the cube of all diameters divided by the sum of the square of all diameters. This yields a characteristic droplet diameter that has a volume-to-surface-area ratio equal to the volume-to-surface-area ratio of the entire spray. This diameter is particularly important in gas turbine evaporative fogging system applications because the mass transfer happens at the interface of the droplets and the surrounding air (i.e. at the droplet surface). To enhance the evaporation of a population of droplets, one has to maximize the active surface areas and minimize the internal volumes.
Other significant results that could show important trends include the arithmetic mean diameter, the surface area mean diameter, the volume mean diameter and the relative span diameter that is indicative of the uniformity of the droplet size distribution. Averaging the Dv90 and D32 values may also derive a single representative diameter. This simplistic approach yields a single representative diameter that gives a good indication as to which nozzle is most suitable for gas turbine inlet air fogging. Calculation of the diameter should be a weighted average performed by using the volume concentrations (ppm) derived for each measurement point.
Laser diffraction is ideal for measuring particle size distribution in inlet fogging systems. It is a non-invasive and fast technique that gives accurate and reliable data. This information can be used to optimize the inlet fogging process and increase power augmentation while reducing corrosion in gas turbines.
The work outlined has contributed to the drafting of a measurement protocol and should allow users to accurately compare nozzle systems, nozzle suppliers and gas turbine performance. This will in turn enable users to attain more information about the physics and engineering of the fogging process, resulting in more efficient and more powerful gas turbines.
1 M. Chaker, C.B. Meher-Homji, and T. Mee, “Inlet fogging of gas turbine engines — Part B: Fog droplet sizing analysis, nozzle types, measurement and testing,” Proceedings of ASME Turbo Expo 2002.
2 Combustion Turbine Inlet Air Conditioning Equipment, Appendix A: “Method of Test for High Pressure Water Atomizing Nozzles for Evaporative Cooling and Humidification,” PTC-51 ASME Standard, June 2003.
Mustapha A. Chaker, Ph.D, is Director, Research and Development at Mee Industries Inc., where he directs a research program relating to high-pressure gas turbine inlet fogging technology. Dr. Chaker has a B.S. and M.S. degree in Physics, and a Ph.D. in engineering sciences from the University of Nice-Sophia Antipolis, France.
Dr. Paul Kippax has a degree in Chemistry and a PhD in Colloid Science, from Nottingham University in the UK. Kippax joined Malvern Instruments in 1997 working on the application of acoustic spectroscopy to the sizing of concentrated dispersions. Since 2002 he has been Product Manager for Diffraction Products.