An Introduction to Shock Waves

When you think of shock waves, I would wager that you picture a supersonic jet zooming past overhead. Or maybe you have experienced the famous (or infamous) “sonic boom” that accompanies shock waves attached to airplane engines. The engineering challenges associated with the often-troublesome behavior of shock waves is present in all scales, from carefully designing the bodywork of the aforementioned fighter jets, to the equally intricate details of flow passages and blade design in turbomachinery. The first step in taking into account the effect of shock waves is to understand what they are. In this post we will be reviewing a short introduction into what shock waves are and a few applications where they might be relevant.

Figure 1: Schlieren image showing the shock waves of a supersonic jet
Figure 1: Schlieren image showing the shock waves of a supersonic jet. Source

What are shock waves?

Shockwaves are non-isentropic pressure perturbations of finite amplitude and from the second law of thermodynamics we can say that shockwaves only form when the Mach number of the flow is larger than 1. We can distinguish between normal shocks and oblique shocks. In normal shocks, total temperature is constant across the shock, total pressure decreases and static temperature and pressure both increase. Across oblique shocks, flow direction changes in addition to pressure rise and velocity decrease. Read More

Cooling Methods in Turbine Blades

Turbine components are placed right after the combustor and are therefore, subject to the highest temperatures in an engine. The turbine blades are directly in the line of fire (so to speak) of these incredibly high temperatures. Higher temperatures yield higher cycle efficiencies, meaning that the limit on efficiency for a cycle is determined by turbine materials. The current state of the art materials can only give so much heat resistance capacity, which makes blade cooling essential. In this post we’ll be taking a look at the various cooling methods that exist for turbine blades, and the tools to design them.

Figure 1: High pressure turbine guide vane with cooling holes
Figure 1: High pressure turbine guide vane with cooling holes. Source

How important is cooling to the efficiency of gas turbine engines?

In a word, very.  Let’s look at an example to better explain.  Our fictitious engine without cooling has an overall pressure ratio of 40 where the maximum allowable turbine entry temperature (TET) is at 1498 K, yielding a thermal efficiency of 33%. When compared to a turbine with cooling, TET can be increased to 1850 K, yielding a thermal efficiency of 38%. This is an 8% increase in efficiency via the addition of cooling. In order to achieve good thermal efficiency in our cycles, turbine components must be cooled!

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E-Turbos: The Future of Turbocharger Technology

The Achilles heel of turbochargers has always been the time between pressing your foot to the gas pedal and waiting for the engine to respond with the desired power. This lapse in engine response, commonly termed turbo lag, is what has hindered turbochargers from delivering optimal performance. The aim of a turbocharger is to provide more power, better efficiency and less lag in power delivery. Engine efficiency is becoming more important than ever before, leading to the development of smaller engines. However, the power requirements are not decreasing which means the loss in engine displacement from small designs must be picked up with alternative technologies, such as turbochargers, which can help improve power delivery and fuel economy.

Figure 1: Garrett Motion electric turbocharger due for production in 2021. Source

Electric turbochargers (e-turbos) provide a solution to eliminating turbo lag while adding additional performance benefits. This allows for larger turbocharger designs which can provide larger power and efficiency gains, stay cooler over longer periods of use, and drastically improve engine responsiveness. Garrett Motion are developing e-turbos for mass market passenger vehicles set for launch in 2021, with a claimed fuel efficiency improvement of up to 10%. When used on diesel engines, this e-turbo could be up to a 20% reduction in NOx emissions. In most cases, fuel efficiency will be improved by about 2 – 4%. Other manufacturers such as Mitsubishi and BorgWarner are already developing their own electric turbos and are expected to have announcements in the near future matching the trend in e-turbo development.

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Basics of Steam Turbine Design

Steam turbines account for more than half of the world’s electricity production in power plants around the world and will continue to be the dominant force in electricity power generation for the foreseeable future. The enhancement of steam turbine efficiency is increasingly important as the urgency to reduce CO2 emissions into the atmosphere is a problem at the forefront of power production. Increasing efficiency in steam turbines, and other components of power plants, will help meet the growing demands for electricity worldwide while reducing harmful greenhouse emissions.

Figure 1 Steam Turbine with Long Last-Stage Blades
Figure 1. Steam Turbine with Long Last-Stage Blades. Source

Steam turbines are used in coal-fired, nuclear, geothermal, natural gas-fired, and solar thermal power plants. Also steam turbines are increasingly needed to stabilize fluctuating power demands from solar and wind power stations as renewable energy sources grow worldwide. The current emphasis on steam turbine development is for increasing efficiency, mainly by increasing steam turbine capacity, as well as increasing operational availability, which translates to rapid start up and shut down procedures.  Read More