Evolution of Reverse Engineering

Introduction

In today’s intensely competitive global market, product enterprises are constantly seeking new ways to shorten lead times for new product developments that meet all customer expectations. In general, product enterprise has invested in CAD/CAM, rapid prototyping, and a range of new technologies that provide business benefits. Nowadays, reverse engineering (RE) is considered one of the technologies that provide business benefits by shortening the product development cycle [1]. Figure 1, shows how reverse engineering can close the gap between what is “as designed” and what is “actually manufactured” [1].

Product Development
Figure 1. Product Development Cycle. SOURCE: : [1]
Reverse engineering (RE) is now recognized as an important factor in the product design process which highlights inverse methods, deduction and discovery in design. In mechanical engineering, RE has evolved from capturing technical product data, and initiating the manual redesign procedure while enabling efficient concurrency benchmarking into a more elaborated process based on advanced computational models and modern digitizing technologies [2]. Today the application of RE is used to produce 3D digital models of various mechanical worn or broken parts. The main steps in any reverse engineering procedure are: sensing the geometry of the existing object; creating a 3D model; and manufacturing by using an appropriate CAD/CAM system [2]. Read More

Inducer Design Considerations and its Effect on Turbopump Cavitation

Why Use an Inducer?

Suppose you want to build a turbopump to increase the pressure of your working fluid. However, you find that the fluid that you are working with keeps vaporizing in the impellers you design, causing all sorts of performance issues. What can you do in this case? One solution is to design an inducer for your turbopump.

Axial inducers are used in turbopumps upstream of the impeller to avoid cavitation, reduce the inlet pressure requirement, and/or allow for operation at higher turbopump rotational speeds for a given inlet pressure [1]. For a turbopump to function properly, the inlet pressure of the pump must be high enough to avoid cavitation. Cavitation is a phenomenon where vapor bubbles which form in the flowing fluid collapse suddenly – potentially causing surface damage of the impeller, performance degradation, as well as catastrophic failure.

The cavitation phenomenon can be visualized in the below image. The inlet flow (flowing from the left side of the image) hits a blunt body in the fluid channel. This causes the pressure to locally drop and vapor bubbles to form. As the fluid continues to flow (towards the right side of the image), the vapor bubbles collapse once the fluid pressure has sufficiently increased.

Formation of vapor bubbles in cavitating fluid flow
Figure 1: Formation of vapor bubbles in cavitating fluid flow

Now that we understand the problem, how can we make sure these cavitation effects won’t happen in our pump? To predict when cavitation will occur, two parameters are commonly used. The available net positive suction head (NPSHa) describes how much greater the local inlet static pressure is relative to the local inlet vapor pressure. Essentially, NPSHa indicates whether the conditions for cavitation to occur are met. The required net positive suction head (NPSHr) describes the inlet head corresponding to a certain drop in performance capability. A typical NPSHr parameter is the standard 3% NPSH (NPSH3) which describes the inlet pressure corresponding to a 3% drop in head rise capability of the pump at a particular flow rate. Generally, NPSHr is measuring whether there is enough cavitation present to cause a noticeable decrease in the pump’s performance. If the NPSHr is much greater than NPSHa, then significant performance decreases due to cavitation may occur. That is to say, when the available net positive suction head is insufficient, bad things can happen, ranging from performance degradation to outright damage and failure.

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When to Use 1D Vs. 3D Simulation

Today’s simulation and analysis (S&A) tools allow engineers to study and verify system/machine properties and visualize the aerodynamic, thermodynamic, structural, and other physical properties without having to build a physical prototype. We can perform cooling secondary flow systems analysis in a gas turbine; a detailed performance study for a supercritical CO2 turbine/compressor; predict cavitation for industry a water pump/rocket turbopump; and so many more. Products and machines are becoming more and more complex. Unfortunately, engineers only run a handful of designs through the S&A process, due to the cost associated with limited computer resources and the time required to run simulations and to create complex 3D models of designs. Furthermore, verification and certification of system designs are often done using actual hardware—a costly and time-consuming endeavor. Considering these aspects, 1D and 3D simulations are significantly important. However, engineers need to determine the trade-off between 1D and 3D simulation.

AxSTREAM and STAR-CCM
Figure 1 AxSTREAM Platform with Modules from 0D to 3D including seamless geometry import into STAR-CCM+

1D Simulation

Imagine what’s required to generate one 3D design for a gas turbine secondary cooling flow system, and multiply it by 1,000 design alternatives. Even if we were to only use conceptual CAD models, this project would require extraordinary computing power and data storage—not to mention simulation and design expertise.

And so, even with the movement to bring more cloud-based S&A tools to market, resources required for 3D modeling will still result in very few designs being extensively explored, thanks to their complexity. Detailed low-dimensional models of system behavior can provide valuable insights into system performance and function thus guiding the design process. Read More

Drilled Nozzle Application in Supersonic Turbines

Most designers associate drilled nozzles in turbomachinery with something exotic, uncharted, and specific only to a minuscule amount of high-loaded turbines operating with a high-pressure drop. Meanwhile, many engineers are not aware that this nozzle design has been applied since the very first turbomachines.

Karl Gustaf Patrik de Laval patented a turbine with asymmetric convergent-divergent nozzles in 1888. At that time the shape of the nozzle allowed him to reach more effective kinetic energy transformation and have an entirely new level of turbine performance.

Figure 1 - Laval Turbine with Drilled Nozzles
Figure 1 – Laval Turbine with Drilled Nozzles. Source

Over a hundred years later, drilled nozzles (or asymmetric nozzles, Laval’s nozzles) have been extensively used in rocket engines, flying vehicles, driving turbines, ORC turbines, and other units for which low cost and weight-dimension constraints play an important role.

Despite the wide application range of turbines with these nozzles, each has its own specific features.

Drilled Nozzles

The main characteristics of drilled nozzles in a turbine (Fig. 2) are the partial admission input, high heat drop per first stage, low reaction, and a low number of stages.

Fig. 2 - Turbine with Drilled Nozzles Flow Path in AxSTREAM
Fig. 2 – Turbine (with Drilled Nozzles) Flow Path in AxSTREAM®

For these turbines, the most critical point during the design process is the first nozzle design. The first supersonic nozzle provides the throughput of the turbine. The main kinetic energy transformation and the main portion of the available isentropic heat drop relates to the first nozzles. As a result, the Mach number at the outlet section of nozzles can reach 3.0 and even be higher. To operate in such regimes, the convergent-divergent vane channels are preferable. Read more

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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Turbo-compressor Technologies for Aviation Fuel Cell Systems: Operational Requirements and Development Trends

Introduction

Fuel cells are an important driver in the current energy system landscape with significant impact on the technology base and economic growth. Global fuel cell system shipments saw a 10% increase in 2020, totaling 1.3GW. The transport sector continues to lead with a growth of 25% on the number of units shipped globally.

The recent years have seen the launch of many projects aimed at the development of fuel cell systems for aviation powerplants. In this context, the effective integration of turbomachinery components is key in driving the overall performance and the economic viability of this technology. These aspects are the topic of this blog.

Fuel Cell Technology

Fuel cells are devices which convert the chemical energy of a fuel directly into electricity by electrochemical reactions. A fuel cell element has a matching pair of electrodes (anode and cathode) separated by an electrolyte. An appropriate flow of fuel (e.g. hydrogen) and oxidizer (frequently oxygen) is delivered to the electrodes: the resulting reaction produces electricity and water plus an amount of heat. The simplicity of this process is shown in Figure 1.

Fuel Cell Conceptual Scheme
Figure 1. Fuel Cell Conceptual Scheme (Source).

There are many advantages: efficiency, reliability, low noise, and compactness, all while implementing an environmentally progressive solution. The application potential is also very diversified, sometimes in very critical fields.

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Enjoy Tuning Your Simulations? LOOK AWAY!

This is an excerpt from the Siemens Blog. You can read the full version here.

Originally written By Chad Custer  on March 11, 2020

Turbomachinery CFD in STAR-CCM+

We live in the day of automatic. It seems like every day there is a new task that can be handled automatically. Smart thermostats make sure the house is comfortable when returning from work. Home lighting themes customize an ambiance based on who is home and the time of day. Automatic delivery of your household goods ensures you’re never out of toothpaste. The automatic potato peeler saves you from having to peel potatoes by hand…ok, well maybe that’s going too far. The point is that technology is great for taking care of tasks based on rules or schedules so that you can focus on more important things.

The pervasiveness of automation makes it even more annoying if once you get to work and sit down to run a simulation, you need to fine-tune settings and fiddle with parameters. Or, even worse, if it’s necessary to babysit a challenging case to make sure it runs as expected. Now, there likely was a time in your professional life when diving head-first into the details of flow solvers was interesting and, dare I say, fun. I know it was for me and if you’re reading this blog, then I bet it was for you too 😊. In fact, when it’s possible to set aside time to dig into some new and different case, it’s still fun to research the best methods to use and how to apply them.

Daily work is different, though. Timelines are always shorter. More data is always needed. Your attention is always split between half-a-dozen urgent tasks, not to mention the ever-growing list of work that you keep meaning to get to once you have time. Forget tuning simulation methods for each case, you need to set up the case once knowing that it will run quickly, reliably and give accurate answers. Read More

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

Scaling and Trimming in Axial Compressors and Fans

Introduction

Despite the deepening understanding of the essence of gas-dynamic processes and the development of computational methods, simpler design methods such as scaling and trimming remain in demand in turbomachinery engineering. The main advantage of these approaches over design from scratch is simplicity and its inexpensive nature due to the small-time expenditure and lower demand from computational resources. Good predictive accuracy of the performance and efficiency of the resulting machines is based on the use of an existing machine with well-known characteristics as a prototype.

Conversely, using the prototype imposes restrictions on the use of scaling and trimming methods. It is almost impossible to get a new design with pressure and efficiency higher than that of the prototype. Also, in cases where it is required to obtain performance that is significantly different from the prototype, the inherent reliability of the original prediction may be insufficient.

Scaling Method

Easy to apply and general, valid, scaling laws are needed for design and application engineers. The scaling laws are needed for the purposes of:

  1. Predicting the full-scale performance machine from model test data obtained from a scaled machine
  2. Obtaining a family of machines with different performances on the basis of one well-tested machine

­

Experimental performance and efficiency testing on a full-size model of large machines such as fans to ventilate tunnels and mines or to move combustion air and smoke gas in power plants may be impractical due to the high energy costs and geometric limitations of the experimental stand. In these cases, a scale model is used. And although complete similarity is not maintained, for example, in terms of the Reynolds number, the correction factors in most cases are well known and the prediction accuracy is high.

The method involves the implementation of the flow path of the designed fan or compressor on a scale to the prototype. This means that all linear dimensions (e.g. diameters, blade chord, axial length, etc.) must be multiplied by the scaling factor (SF). The angular dimensions (e.g. blade angles at inlet and outlet, stagger angle, etc.) remain unchanged.

When scaling, it is assumed that parameters such as Pressure Ratio, circumferential velocity (U), and axial velocity (Cz) are equal for the designed machine and the prototype. Thus:

Trimming and Scaling Formula 1

The condition of equality of the Reynolds criteria is not ensured, since the designed compressor and the prototype do not have the same diameters of the rotor with the equality of other parameters that determine the number of Rew. This design guarantees the practical accuracy of the calculated characteristics, provided that the gas movement in the flow path is turbulent. It is known that for “physical” values of the Reynolds numbers

Trimming and Scaling Formula 2

the flow remains turbulent and the inequality of the Reynolds numbers of the designed compressor and the prototype has little effect on the gas-dynamic characteristics.

To determine the efficiency of low-pressure fans, a well-known formula is usually used:

Trimming and Scaling Formula 3

An example of obtaining a stage of an axial compressor by the scaling method is shown in Figure 1.

Figure 1 - Axial Compressor Stage Scaling
Figure 1 – Axial Compressor Stage Scaling

The disadvantages of the scaling method include the need to change the rotor speed. This can be relevant for industrial installations, where the rotation speed is often limited and tied to the frequency of the electrical network current. Additionally, the need to change the overall dimensions can be a limiting factor, especially if it is necessary to increase productivity significantly, and the installation location for the turbomachine is limited. In some cases, maintaining full geometric similarity is impossible for technological or constructive reasons. For example, the minimum value of the tip clearance may be limited by the operating conditions of the rotor (not touching the rotor against the housing) or the impossibility of obtaining a small clearance if the scaling is carried out from a large prototype to a small model.

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Initial Sizing of Centrifugal Fans

Centrifugal fans are a type of turbomachine equipment widely used in all kinds of modern and domestic life. Centrifugal fans were developed as highly efficient machines, and the design is still based on various empirical and semi-empirical rules proposed by fan designers. Due to these various rules, there are different methodologies used to design impellers and other components.

Centrifugal fans consist of an impeller in a casing with a spirally shaped contour, shown in Figure 1 (left side). The air enters the impeller in an axial direction and is discharged at the impeller’s outer periphery. The airflow moves along the centrifugal direction (or radial direction). Centrifugal fans can generate relatively high pressures, as compared with axial flow fans. For axial flow fans, the pressure rise is small, about be few inches of water.

Radial Fan and Static Pressure
Figure 1 Radial Fan and Static Pressure, Shaft Power V/s Volume Flow Curves for Different
Types of Blades

Generally, centrifugal fans have three types of blades: forward blade, backward blade, and radial blade. The characteristic curve of these three kinds of centrifugal fans is shown on the right side in Figure 1.

Sizing Using Cordier Diagram

Centrifugal fans (most turbomachines) can be classified based on specific speed (Ns) and specific diameters (Ds) as shown in Figure 2. Specific speed is a criterion at which a fan of unspecified diameter would run to give unit volume flow and pressure. The correlation for a specific speed and specific diameter can be seen here:

Pump Formula

where, ‘N’ is rotational speed (RPM), ‘Q’ is flow rate (ft3/sec), ‘H’ is head (ft), ‘D’ is diameter (ft) Read More