MEE403 — Applied Engineering Thermodynamics: Lecture Note

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This is the complete lecture note for MEE403 Applied Engineering Thermodynamics. You can read it online below, open it in the full viewer, or download the PDF for offline study. It pairs with the course's video lectures, so you can read and watch together.

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Course code: MEE 403 Course title: Applied Engineering Thermodynamics Course credit load: 2 Units Series 1: Compressors and Gas Turbine Plants • Part 1.1: Reciprocating Compressors and Multistage Compression o Sub Part 1.1.1: Principles of Reciprocating Compression o Sub Part 1.1.2: Single-Stage and Multistage Compression o Sub Part 1.1.3: Intercooling and Multistage Efficiency • Part 1.2: Rotary Compressors: Centrifugal Type o Sub Part 1.2.1: Principles of Centrifugal Compression o Sub Part 1.2.2: Centrifugal Compressor Components and Stages o Sub Part 1.2.3: Performance Characteristics of Centrifugal Compressors • Part 1.3: Rotary Compressors: Axial-Flow Type and Stagnation Properties o Sub Part 1.3.1: Principles of Axial-Flow Compression o Sub Part 1.3.2: Comparing Centrifugal and Axial-Flow Compressors o Sub Part 1.3.3: Stagnation Properties in Compressible Flow • Part 1.4: Simple Gas Turbine Plant o Sub Part 1.4.1: Components and Working Cycle of a Simple Gas Turbine Plant o Sub Part 1.4.2: The Brayton Cycle o Sub Part 1.4.3: Performance and Applications of Gas Turbine Plants Introduction A compressor sits at the heart of almost every industrial process that depends on raised gas pressure, from powering pneumatic tools to feeding a gas turbine's combustion chamber, and the type of compressor chosen for a given duty depends directly on the pressure ratio, flow rate, and continuity of delivery that the application demands.

The aim of this Series is to equip you with an understanding of reciprocating and rotary compressors, the significance of stagnation properties in compressible flow, and the working cycle of a simple gas turbine plant. We shall commence this Series with reciprocating compressors and multistage compression, before turning to centrifugal rotary compressors. We will then examine axial-flow compressors and stagnation properties, and conclude this Series by considering the simple gas turbine plant. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1: describe the operating principle of a reciprocating compressor, • SLO 1.2: explain the purpose and benefit of multistage compression with intercooling, • SLO 1.3: describe the working principle of a centrifugal compressor, • SLO 1.4: analyse the performance characteristics of a centrifugal compressor, • SLO 1.5: describe the working principle of an axial-flow compressor, • SLO 1.6: define stagnation properties and explain their significance in compressible flow, • SLO 1.7: describe the components and working cycle of a simple gas turbine plant, • SLO 1.8: analyse the performance of a gas turbine plant using the Brayton cycle, 1.1 Reciprocating Compressors and Multistage Compression 1.1.1 Principles of Reciprocating Compression A reciprocating compressor raises gas pressure by drawing gas into a cylinder through an inlet valve on the piston's suction stroke, then compressing that trapped gas as the piston reverses direction, finally forcing it out through a delivery valve once the cylinder pressure exceeds the discharge line pressure, cycle after cycle. The compression process within a single cylinder follows a roughly polytropic path on a pressure-volume diagram, with the actual pressure rise achieved in one stroke limited by the mechanical strength of the cylinder, the temperature rise the trapped gas can tolerate, and the volumetric efficiency that a single stage can practically sustain before clearance losses erode delivery further.

Worked example: A small workshop air compressor drawing atmospheric air and delivering it at several bars into a storage receiver is a familiar example of single-cylinder reciprocating compression in everyday use. Application: Recognising the piston-and-valve cycle that underlies reciprocating compression helps explain why delivery is inherently pulsating rather than continuous, unlike the steady flow a rotary machine produces. Fill in the blank: A reciprocating compressor forces trapped gas out through a delivery valve once cylinder pressure exceeds the ________ line pressure. 1.1.2 Single-Stage and Multistage Compression Single-stage compression raises gas pressure from suction to final delivery in one continuous cylinder pass, an arrangement well suited to modest pressure ratios but increasingly inefficient and thermally demanding as the required overall pressure ratio grows larger. Multistage compression instead divides the same overall pressure rise across two or more cylinders arranged in series, each cylinder handling only a portion of the total ratio, so that the gas temperature reached in any single cylinder stays within a manageable and mechanically safe limit throughout the whole process. Worked example: Compressing atmospheric air to twenty bars in a single cylinder would raise its temperature to an impractical level, whereas splitting the same rise across three cylinders keeps each stage's discharge temperature comfortably moderate. Application: Choosing multistage compression over a single stage becomes the practical necessity once the required overall pressure ratio exceeds what one cylinder can safely and efficiently deliver. Fill in the blank: Dividing an overall pressure rise across two or more cylinders arranged in series is called ________ compression. 1.1.3 Intercooling and Multistage Efficiency Intercooling passes the partially compressed gas through a heat exchanger between successive cylinders, removing the heat added during the preceding stage before the gas enters the next cylinder, so that each stage effectively begins its own compression from a cooler starting temperature. Cooling the gas between stages reduces the specific work a multistage compressor must supply overall, since a cooler gas occupies less volume for a given pressure and therefore requires less compression work in every stage that follows, an effect that becomes more pronounced the more stages are intercooled.

Worked example: An intercooler fitted between the low-pressure and high-pressure cylinders of a two-stage air compressor typically returns the partially compressed air close to its original inlet temperature before the second stage begins. Application: Fitting effective intercooling between stages is one of the most direct ways an engineer can reduce a multistage compressor's overall running cost for a given duty. Fill in the blank: Removing heat added during a preceding compression stage before the gas enters the next cylinder is called ________. Figure 1.1: A two-stage reciprocating compressor with an intercooler. Pilot questions 1.1 1. Describe the operating cycle of a reciprocating compressor. (Linked to SLO 1.1) 2. Explain why delivery from a reciprocating compressor is pulsating rather than continuous. (Linked to SLO 1.1) 3. Distinguish between single-stage and multistage compression. (Linked to SLO 1.2) 4. Explain why multistage compression becomes necessary at higher pressure ratios. (Linked to SLO 1.2) 5. Explain the purpose of intercooling in a multistage compressor. (Linked to SLO 1.2)

1.2 Rotary Compressors: Centrifugal Type 1.2.1 Principles of Centrifugal Compression A centrifugal compressor raises gas pressure by accelerating the gas radially outward through a rotating impeller, converting the kinetic energy this rotation imparts into a rise in static pressure as the fast-moving gas is subsequently slowed and diffused within a surrounding stationary casing. Unlike a reciprocating machine, a centrifugal compressor delivers gas continuously rather than in discrete pulses, since the impeller simply keeps rotating and accelerating a steady stream of gas outward, an arrangement especially well suited to applications demanding large, steady volumetric flow rates. Worked example: A turbocharger's compressor wheel, spinning at very high speed to accelerate intake air outward before it is diffused and fed into an engine, is a familiar small-scale example of centrifugal compression. Application: Recognising that a centrifugal compressor's delivery is continuous rather than pulsating explains why it suits applications demanding large, steady flow far better than a reciprocating machine of comparable duty. Fill in the blank: A centrifugal compressor converts kinetic energy imparted by a rotating impeller into a rise in ________ pressure. 1.2.2 Centrifugal Compressor Components and Stages The impeller draws gas in axially near its centre and accelerates it radially outward through a series of curved vanes, after which a surrounding diffuser, a set of stationary passages of gradually increasing cross-section, slows the fast-moving gas and converts much of its velocity into additional static pressure rise. A single impeller stage can only achieve a limited pressure ratio before its efficiency begins to fall away, so a centrifugal compressor intended for a higher overall pressure ratio typically arranges several impeller stages in series, each stage's discharge feeding directly into the next stage's inlet. Worked example: A multistage centrifugal air compressor supplying a large industrial plant commonly arranges three or four impellers on a single shaft, each stage raising the pressure a further increment before the gas reaches the next. Application: Arranging several impeller stages in series, rather than relying on one oversized impeller, is the standard route to achieving a higher overall pressure ratio in a centrifugal machine.

Fill in the blank: Stationary passages of gradually increasing cross-section that slow gas leaving the impeller and raise its static pressure are called a ________. 1.2.3 Performance Characteristics of Centrifugal Compressors Surging occurs when a centrifugal compressor is operated at too low a flow rate for a given rotational speed, causing the flow through the impeller to break down and reverse momentarily before re-establishing itself, a cycle that repeats rapidly and can damage the machine if allowed to persist. A compressor's performance is normally summarised on a characteristic map plotting delivery pressure against volumetric flow rate for a family of constant-speed curves, with a surge line marking the minimum flow boundary below which stable operation is no longer possible at each

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