chapter 14: turbomachinery

Chapter 14: Turbomachinery

Eric G. PatersonDepartment of Mechanical and Nuclear Engineering

The Pennsylvania State University

Spring 2005

Chapter 14: TurbomachineryME33 : Fluid Flow 2

Note to InstructorsThese slides were developed1, during the spring semester 2005, as a teaching aid for the

undergraduate Fluid Mechanics course (ME33: Fluid Flow) in the Department of Mechanical and Nuclear Engineering at Penn State University. This course had two sections, one taught by myself and one taught by Prof. John Cimbala. While we gave common homework and exams, we independently developed lecture notes. This was also the first semester that Fluid Mechanics: Fundamentals and Applications was used at PSU. My section had 93 students and was held in a classroom with a computer, projector, and blackboard. While slides have been developed for each chapter of Fluid Mechanics: Fundamentals and Applications, I used a combination of blackboard and electronic presentation. In the student evaluations of my course, there were both positive and negative comments on the use of electronic presentation. Therefore, these slides should only be integrated into your lectures with careful consideration of your teaching style and course objectives.

Eric PatersonPenn State, University ParkAugust 2005

1 These slides were originally prepared using the LaTeX typesetting system (http://www.tug.org/) and the beamer class (http://latex-beamer.sourceforge.net/), but were translated to PowerPoint for wider dissemination by McGraw-Hill.


Objectives

Identify various types of pumps and turbines, and understand how they workApply dimensional analysis to design new pumps or turbines that are geometrically similar to existing pumps or turbinesPerform basic vector analysis of the flow into and out of pumps and turbinesUse specific speed for preliminary design and selection of pumps and turbines


Categories

Pump: adds energy to a fluid, resulting in an increase in pressure across the pump.

Turbine: extracts energy from the fluid, resulting in a decrease in pressure across the turbine.


Categories

For gases, pumps are further broken down intoFans: Low pressure gradient, High volume flow rate. Examples include ceiling fans and propellers.Blower: Medium pressure gradient, Medium volume flow rate. Examples include centrifugal and squirrel-cage blowers found in furnaces, leaf blowers, and hair dryers.Compressor: High pressure gradient, Low volume flow rate. Examples include air compressors for air tools, refrigerant compressors for refrigerators and air conditioners.


Categories

Positive-displacement machinesClosed volume is used to squeeze or suck fluid. Pump: human heartTurbine: home water meter

Dynamic machinesNo closed volume. Instead, rotating blades supply or extract energy.Enclosed/Ducted Pumps: torpedo propulsor Open Pumps: propeller or helicopter rotorEnclosed Turbines: hydroturbineOpen Turbines: wind turbine


Pump Head

Net Head

Water horsepower

Brake horsepower

Pump efficiency


Matching a Pump to a Piping System

Pump-performance curves for a centrifugal pumpBEP: best efficiency pointH*, bhp*, V* correspond to BEPShutoff head: achieved by closing outlet (V=0)$Free delivery: no load on system (Hrequired = 0)


Matching a Pump to a Piping System

Steady operating point:

Energy equation:


Manufacturer Performance Plot


Pump Cavitation and NPSH

Cavitation should be avoided due to erosion damage and noise.

Cavitation occurs when P < Pv

Net positive suction head

NPSHrequired curves are created through systematic testing over a range of flow rates V.


Dynamic Pumps

Dynamic Pumps include centrifugal pumps: fluid enters axially, and is discharged radially.

mixed--flow pumps: fluid enters axially, and leaves at an angle between radially and axially.

axial pumps: fluid enters and leaves axially.


Centrifugal Pumps

Snail--shaped scroll

Most common type of pump: homes, autos, industry.


Centrifugal Pumps


Centrifugal Pumps: Blade Design



Side view of impeller blade. Vector analysis of leading and trailing edges.



Blade number affects efficiency and introduces circulatory losses (too few blades) and passage losses (too many blades)


Axial Pumps

Open vs. Ducted Axial Pumps


Open Axial Pumps

Propeller has radial twist to take into account for angular velocity (=r)

Blades generate thrust like wing generates lift.


Ducted Axial Pumps

Tube Axial Fan: Swirl downstream

Counter-Rotating Axial-Flow Fan: swirl removed. Early torpedo designs

Vane Axial-Flow Fan: swirl removed. Stators can be either pre-swirl or post-swirl.


Ducted Axial Pumps: Blade Design

Absolute frame of reference Relative frame of reference


Dimensional Analysis

analysis gives 3 new nondimensional parameters

Head coefficient

Capacity coefficient

Power coefficient

Reynolds number also appears,but in terms of angular rotation

Reynolds number

Functional relation is

Head coefficient

Power coefficient



If two pumps are geometrically similar, and

The independent ’s are similar, i.e., CQ,A = CQ,B

ReA = ReB

A/DA = B/DB

Then the dependent ’s will be the sameCH,A = CH,B

CP,A = CP,B



When plotted in nondimensional form, all curves of a family of geometrically similar pumps collapse onto one set of nondimensional pump performance curves

Note: Reynolds number and roughness can often be neglected,


Pump Specific Speed

Pump Specific Speed is used to characterize the operation of a pump at BEP and is useful for preliminary pump selection.


Affinity Laws

For two homologous states A and B, we can use variables to develop ratios (similarity rules, affinity laws, scaling laws).

Useful to scale from model to prototypeUseful to understand parameter changes, e.g., doubling pump speed (Ex. 14-10).

chapter 14: turbomachinery

Documents

fluid flow

chapter of fluid mechanics

high volume flow rate

medium volume flow rate

low volume flow rate

enclosedducted pumps

existing pumps

selection of pumps