Gas Turbine Cooling System Design Procedures

Introduction

State-of-the-art gas turbine engines usually work under extremely high temperatures. This is directly related to efficiency of the gas turbines – in order to receive the maximum thermodynamics value, it is necessary to increase the gas temperature after the combustion chamber. Engine temperature can be higher than blades’ metal temp up to 500-600 K. Blades, nozzles, and the GT details are manufactured with special heat-resistant steels and in some cases, they require a special coating. That allows them to resist turning into liquid metal under these working temperatures like the T-1000 did in the “Terminator 2: Judgment Day” movie even under high temperatures :).

Picture 1 – T-1000 from Terminator 2
Picture 1 – T-1000 from Terminator 2. Source

However, metal has the property of “creep” – this is the tendency of hard metal to move slowly or permanently deform under stress. This occurs as a result of prolonged exposure to high stresses above the yield point, especially when exposed to high temperatures. Obviously, the solution to this problem is a cooling system for heat-stressed parts, which has allowed the gas temperature to increase by 600 K compared with uncooled machines. Since the gas turbines usually work with air, the simplest way to cool the system is by using this. Typically, the air exhausts to different parts of the compressors and is supplied to the cooling paths and blades which influence the thermodynamics efficiency of the gas turbine engine. Thus, it is crucial to ensure enough cooling to remove the heat on the one hand and on the other hand – to receive the lowest amount of air which requires cooling. Read More

Aircraft Life Support Systems Part 2: Water and Waste System

INTRODUCTION

In the aircraft industry, several systems are designed to provide safety and comfort for the crew and passengers.

Regarding comfort, the water and waste system is designed to provide water for galleys and lavatories. Fresh water is stored and distributed while a different system deals with wastewater. That system includes a thoughtful engineering method to dispose of the different wastes that could occur during the flight.

OVERVIEW

Water must be supplied to different parts of the plane during flight. This water is kept in a tank in the compartment aft of the bulk cargo compartment. The whole system is made up of a passenger water system that stores, delivers, monitors and controls drinkable (potable) water for the galley units and lavatory sink basins.

In this blog, we are going to focus more specifically on the 737-classic model from Boeing.

Figure 1-Representation of different parts of the water and waste system
Figure 1-Representation of different parts of the water and waste system

The 3 main achievements of the water and waste system are the following:

  • Filling the water tank on land
  • Providing water during the flight
  • Storing toilet waste

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The water and waste system is made up of:

  • Potable water system aims to deliver fresh water to every needed part in the plane (including every component between the water tank and sinks)
  • Water tank pressurization system focuses on the pressurization of the water tank and air dealing with the tank (including air compressor, pressure regulator filter, pressure relief valve)
  • Wastewater system focuses on water related to lavatory and sinks / galleys wastewater (including drain masts)
  • Toilet system includes components related to flushing and toilet water (including waste tank)

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The water tank has a capacity of 34 gallons (about 0.15 m3). The water system in the plane needs to be pressurized for altitude just like the cabin, so it gets pressurized by an air inlet (linked to the pneumatic system). Therefore, the water quantity should not exceed 30 gallons (about 0.13 m3). Read More

Aircraft Life Support Systems Part 1: Oxygen System

INTRODUCTION

In the aircraft industry, several systems are designed to provide safety and comfort for crew and passengers while traveling. Oxygen gets rarified with altitude, so life support is a very important system

The cabin is pressurized in order to provide breathable air, but reaching a sea level pressure is not advisable since it would lead to a significant pressure differential between the aircraft exterior and the cabin interior. This difference could damage the aircraft structure.

Additionally, the cabin altitude is different from the flight altitude. In fact, the cabin altitude corresponds to the one reached according to the cabin pressure. Usually a commercial flight cruises at an altitude of 35,000 ft, but thanks to the pressurization system, the cabin altitude is around 6,000-8,000 ft.  Indeed, the oxygen system provides breathable oxygen to the crew and passengers if any problem were to occur during the flight.

AIRCRAFT EMERGENCY OXYGEN SYSTEM:

In a normal situation, a bleed air system is used to provide fresh air throughout the flight duration. The air is hot and must be cooled and pressurized to make it breathable.  In the event of an emergency, the plane is already equipped with oxygen systems which are linked to passengers and cabin crew through masks. In fact, there are two oxygen systems on board. One designed for the crew, and the second for the passengers.

If the cabin pressure drops making cabin altitude about 14,000 ft, the emergency system are be triggered. The emergency system provides oxygen to passengers for 15 to 20 minutes, and for the crew members for around 30 minutes. This is enough time for the aircraft to descend to a lower altitude and being the cabin altitude to a safe breathable level.

Here, the crew oxygen system schematic of the Boeing 737 class is shown in Figure 1.

Figure 1-Crew oxygen system
Figure 1-Crew oxygen system

The main challenges of oxygen equipment are:

  • Fitting the dimensions of the plane
  • Secure (no leakage for example)
  • Responsive (to cabin pressure and cabin altitude)
  • Easy for passengers to use the oxygen system through the deployed masks quickly, before the effects of altitude are felt:
  • At 25,000 ft: a person has 3 minutes of consciousness
  • At 41,000 ft: a person has 30 seconds of consciousness

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FLIGHT CREW OXYGEN

The flight crew oxygen should be designed and made with a lot of care, because if any trouble occurs during the flight, the crew must be able to handle the situation and take the airplane and its passengers down safely. Read More

The History of Turbochargers, Part 3

Hello! Welcome back to this third and final installment in our “History of the Turbocharger” Series. If you haven’t already, you can read the previous installments by clicking the links below:

Now, let’s see how the turbocharger went from an ace-in-the-hole for aircraft engines during World War II, to the go-to way to crank out horsepower in small engines.

Up until World War II, turbochargers were not a common sight in cars, and certainly not the most popular option for adding forced induction to an engine. Even following the war, some of the most notable post-war aircraft relied on piston engines as opposed to the modern turbojet engine, did not use turbochargers. Most R&D efforts for military aircraft propulsion was moving away from piston engines, and where piston engines were being used, they didn’t have turbos.

Take, for example, the Corvair B36. This behemoth of an airplane was adopted by the US Air Force for a short period of time after the war, but before the much more famous B52 Stratofortress was adopted. This gargantuan plane made use of a Pratt and Whitney radial engine similar to (although much larger than) the engines used in other US warplanes during World War II. Much like the other engines used by warplanes, these engines were typically not turbocharged, instead used geared superchargers to force more air into the 6(!) propeller engines.

B-36aarrivalcarswell1948
A Corvair B36 Peacemaker, which dwarfed the already big B29 Superfortress. By the time the Peacemaker Flew, piston engines were already being considered obsolete for most military and aviation applications.

From the get-go, this engine was quite dated, as the piston engines were maintenance heavy, and the unusual engine and propeller configuration gave the plane reliability issues. Additionally, the Peacemaker was retrofitted with 4 jet engines for use in takeoff as well as speed over a target to reduce the likelihood of being struck by enemy fire. It wasn’t long however, before the turbojet-powered B52 we all know and love was adopted. The B36 was more or less forgotten as a massive placeholder for the US Air Force for a short time following World War II. Read More

Performance Testing of Axial Compressors

Performance testing is a key part of the design and development process of advanced axial compressors.  These are widely used in the modern world and can be found in nearly every industry, and include the core compressor for aeropropulsion turbofan engines, as well as aeroderivative gas turbine engines for power generation.  An example of this are the turbine engines shown in Figure 1 and 2, which feature an industrial gas turbine and a high bypass ratio turbofan engine with a multistage high-pressure core compressor. The development time of these machines can involve numerous expensive design-build-test iterations before they can become an efficient and competitive product. This places a great importance on the accuracy of the data taken during the performance tests during the development of the compressor since the test data taken is often used to anchor the loss models within the design tools. Modern axial compressors typically have high aerodynamic loadings per stage for improved system efficiency and requires precise aerodynamic matching of the stages to achieve the required pressure ratio with high efficiency. Variable geometry inlet guide vanes and stators in the first few stages are typically required to provide acceptable operability while maintaining high efficiency and adequate stall margin.

Industrial gas turbine for power generation.
Figure 1. Industrial gas turbine for power generation. Source
Figure 2. Turbofan engine for aeropropulsion.
Figure 2. Turbofan engine for aeropropulsion. Source

Performance Testing of Axial Compressors

Axial compressors all undergo a thorough design and development phase in which performance testing is vital to their ultimate success as a product. Performance testing during the development phase of these high-power density machines can ensure that the design meets the specified requirements or can identify a component within the turbomachine which falls short of its expected performance, and may require further development, and possible redesign. Performance testing can also ensure that the unit can meet all the conditions specified and not merely the guaranteed condition. Aerodynamic performance testing multistage axial compressors during the early part of development is often done in phases. The development test program is planned and executed with a design of experiments approach and includes varying the air flow and shaft rotational speed as well as the variable geometry schedule in order to fully characterize the compressor. In the first phase, the front block of the compressor is built and tested at corrected (referenced) air flow rate, inlet pressure, temperature and shaft rotational speed. Instrumentation includes utilizing traditional rakes and surveys at the exit, to obtain spanwise distributions of pressure, temperature, and flow angles. Testing in phases is typically done for two reasons. Read More

Anti-Icing Systems in Airplanes: Boeing 737-300/400/500

Through the decades, the aircraft industry always improved their onboard systems to get the best performances, security and comfort. In order to build a lasting travel type, security of the aircraft is one of the main goals for engineers. Due to rough exterior conditions while flying, especially at high altitude, with relative humidity and very low temperatures, the freezing temperature can cause the plane to ice. Ice can have major impacts on the aircraft’s weight and aerodynamical phenomena, – especially the lift – (the lift can decrease to 40% due to ice). Modeling and installing a specific system to prevent ice is a necessity. Therefore, aircraft designers developed an anti-icing system inside the wing to prevent ice.

There are several anti-icing systems on aircraft, mostly depending of the engine’s type. Most of aircrafts use the bleed air system, which consists of using a hot bleed air to warm up the wing leading edge. Another system named de-icing boots system is mostly used on turboprop aircrafts and consists of black rubbers at locations prone to icing which inflate and literally break the ice. Another system is simply an electrical leading edge warm up directly installed in the wing leading edge. Those examples are just an introduction to some anti-icing systems that aircraft industry has develop and are using. Each have pros and cons.

Here, we will focus on the anti-icing system using hot bleed air. This approach is used by the Boeing 737-300/400/500 anti-icing system with hot bleed air warming the leading edges.

Typically, this type of anti-icing system consists of a hot bleed air flow provided by the engine compressor’s stages to warm up the plane’s wing leading edge. The wing anti-icing system is made of two independent pneumatic systems among others, providing hot bleed air from each of the two turbofans separately. The hot bleed air is ducted via the engine bleed valve from the fifth compressor stage. If the pressure isn’t enough, bleed from the ninth compressor stage can additionally be used. Note that the fifth stage bleed air temperature is approximately 340°C and the ninth stage one is approximately 540°C which are too hot to be used in aircraft’s pneumatic systems such as hydraulic pressurization or potable water system pressurization for example. The hot air then runs through a pre-cooler to reduce the temperature to 200°C and this cooled air is distributed via the bleed ducts to consumers like the air conditioning packs for example and the wing anti-icing system. In order to know the moment to use the anti-icing system, the aircraft’s pilots use the visual ice indicator which is situated in the middle beam of the window. Once the probe is icing, the pilots enable the anti-icing system. Hence, hot bleed air is provided to the slates number three, four and five as shown in Figure 1.

Landing Edge Slats
Figure 1 – Leading Edge Slats

Due to the larger diameter and the aerodynamics phenomena, slates number one and two do not need any anti-icing devices. Once the anti-icing system is enabled, the hot bleed air is guided along telescopic pipes then is distributed via piccolo tubes as shown in Figure 2. From there, it exits the piccolo tubes through little holes, warms the wing leading edge and flows out of the wing through exit holes situating on the wing’s lower surface. Read More

Vertical Pumps: What Are They, Where Are They Used and How To Design Them?

Introduction

Vertical pump designs are similar to conventional pumps, with some unique differences in their applications.  Pumps use centrifugal force to convert mechanical energy into kinetic energy and increase the pressure of the liquid. Vertical pumps move liquids in the vertical direction upwards through a pipe. All pumps pressurize liquids, which are mostly incompressible. Unlike compressible gases, it is impossible to compress liquids, therefore the volumetric flow rate can not be reduced. Therefore liquids are transported by pumping and the inlet volume flow rate is equal to the exit volume flow rate.

Vertical centrifugal pumps are simply designed machines, and have similarities to their horizontal counterparts. A casing called a volute contains an impeller mounted perpendicularly on an upright (vertical) rotating shaft. The electric drive motor uses its mechanical energy to turn the pump impeller with blades, and imparts kinetic energy to the liquid as it begins to rotate. These pumps can be single stage or multistage with several in-line stages mounted in series.

The centrifugal force through the impeller rotor causes the liquid and any particulates within the liquid to move radially outward, away from the impeller center of rotation at high tangential velocity. The swirling flow at the exit of the impeller is then channeled into a diffusion system which can be a volute or collector, which diffuses the high velocity flow and converts the velocity into high pressure. In vertical pumps, the high exit pressure enables the liquid to be pumped to high vertical locations. Thus the pump exit pressure force is utilized to lift the liquid to high levels, and usually at high residual pressure even at the pipe discharge.

Applications of Vertical Pumps

An “in line” vertical pump is illustrated in Figure 1 (Reference 1), where the flow enters horizontally and exits horizontally and can be mounted such that the center line of the inlet and discharge pipes are in line with each other.  This is a centrifugal pump with a tangential scroll at the inlet that redirects the flow by 90 degrees and distributes it circumferentially and in the axial direction into the impeller eye. The discharge is a simple volute that collects the tangential flow from the impeller exit, and redirects it into the radial direction.

in line Pump - Figure 1
An “in line” Vertical Pump. Source

Figure 2 shows a vertical pump that has a vertical intake that directs the flow straight into the eye of the pump rotor. At the impeller exit, the tangential flow is collected by a volute and diffused in an exit cone. An elbow after the exit cone redirects the flow into the vertical direction to lift the liquid to the desired altitude. (Reference 2). Read More

Centrifugal Compressors for Fuel Cells

The development of fuel cell technologies and improvements in fuel cells power densities combine to make the use of fuel cells possible in different power sectors as primary or secondary power sources for commercial purpose, residential power requirements, and automobiles, etc. The fuel cell harnesses the chemical energy of a fuel along with an oxidizing agent by converting it into electrical energy through a pair of reactions. For example, in a hydrogen fuel cell, as shown in Figure 1, the hydrogen combines with oxygen from the air to produce electricity and releases water.

Fuel Cell System
Figure 1 Fuel Cell System [1]
The design of a fuel cell system is quite complex and depends on fuel cell types and their applications. With so many possible combinations of fuel cells, this article will not focus on different type of fuel cells, but on Air Management Systems which may significantly affect the overall performance of a fuel cell system.

Air Management Systems

Key sub-systems of any fuel cell system are the fuel processor, fuel cell stack, air management and power management systems. The air management system strongly affects the fuel cell stack efficiency and the power loss of the fuel cells. Therefore, it is necessary to develop a clean, reliable, cost-effective oil-free air system [2].

Major tasks in air management system are Air Supply, Air Cleaning, Pressurization and Humidification.
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Aircraft Fuel Systems

The airplane is a complex technical object. Like a human or other organisms, it consists of numerous vital systems; with one of the more critical ones being the fuel system. It is important part of any vehicle, let alone aircraft, aside from  the newest electric powered vehicles.

An aircraft’s fuel system provides fuel that is loaded, stored, managed and transported to the propulsion system of the vehicle[1, 2]. As aviation fuel is liquid, this system can be considered as hydraulic. Therefore, it’s able to be mapped out and modeled for analysis in a program like AxSTREAM NET™.

The Typical Fuel System of a Narrow-body Passenger Plane

For an example of a conventional aviation fuel system, consider a typical narrow-body airliner with two engines. Some of the popular planes in this category include the Boeing 737, the Tupolev Tu-204, Airbus A320, Comac C919, Sukhoi Superjet 100, Bombardier CRJ, Embraer E-Jet and Mitsubishi Regional Jet[3].

The storage fuel system is shown in figure 1 is for the Boeing 737-300. The fuel is kept in an integral tank that is divided to five separate subdivisions. They are the central, wing (main) and surge tanks[4].

Storage fuel system of a Boeing 737
Figure 1 – Storage Fuel System of a Boeing 737-300 [4]
The hydraulic scheme of the Boeing 737’s fuel system is shown in Figure 2. For fueling and defueling the storage system there are ports on the starboard wing. The system does not have pumps to onboard fuel, so fuel is pumped into the plane via a fuel truck. The other critical part of the fuel system is the line which delivers fuel to the two engines and the auxiliary power unit. In this line there are two boost centrifugal pumps by each engine.
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A Century of Chiller Technology

A convergence of technologies had to occur to make the modern, high-efficiency centrifugal chiller a reality. To appreciate the technology fully, we must go back in history and understand the origins of the air conditioning and refrigeration industry. Along the way, we will find an important diversion in aerospace and the critically important centrifugal compressor. Ultimately, we will find that the modern chiller is a testament to advanced technology that was developed in multiple fields.

Some of the first advances in and applications of modern industrial refrigeration were in the United States. In May 1922, Willis Carrier revealed the “Centrifugal Refrigeration Machine” – a very early incarnation of what we now call a chiller [1]. The first installation went to a Philadelphia candy manufacturer; it’s interesting to know that the birth of modern refrigeration and air conditioning started on a large scale. Back in those days, economy of scale enabled the technology to be developed. It was not until a decade later that the core technology began to be adopted into compact units that could be used in smaller businesses such as boutique shops. It took several more decades for smaller residential air conditioners to take off commercially.

Shown in the photograph below is Carrier’s first centrifugal chiller in his New Jersey factory [1].

First Centrifugal Chiller
Photo from [1]
The size of this machine is evident, as is the fact that its design, at the time, necessitated components be spread out in space for assembly and maintenance. By modern standards, the same footprint space could be used to accommodate a modern chiller with over 500 refrigeration tons in capacity. By comparison the original design has less than 100 refrigeration tons of capacity.

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