Nozzle thrust experiment

QUEEN MARY UNIVERSITY OF LONDON SCHOOL OF ENGINEERING AND MATERIALS DEN-306 AIRCRAFT PROPULSION DEN427/DENM022 ADVANCED GAS TURBINES LABORATORY EXPERIMENT AERO LAB BALCANY Nozzle Thrust Objective : To study the thrust produced by a convergent nozzle operating under both subsonic and choked conditions. (See Lectures 6 and 7).

Apparatus and Instrumentation : Compressed air is delivered from the laboratory supply, via a shut-off valve, to a pressure regulating valve, and one of these valves can be used to control the pressure in the whole of the apparatus up to the inlet chamber of the nozzle, where it is measured by a pressure transducer. After the regulating valve, the volumetric flow rate of the air is measured by a rotameter. The air is fed to the nozzle through a pivoted arm and the small chamber, the bottom of which rests on the pan of some electronic scales. The nozzle has a rounded inlet from the much larger diameter of the chamber to one of about 5.8 mm, and it then tapers over a length of 50mm down to the outlet to atmosphere, which is 3.0 mm in diameter. The bore of the nozzle is not perfectly smooth.

Procedure: 1) Progressively open whichever valve is regulating, recording the indicated flow through the rotameter, the pressure at the nozzle inlet and the weight on the scales, for about a dozen flow rates, repeating as necessary to assess reproducibility. In practice, it will be necessary to check frequently that the weight indicated for zero flow has not altered. This means ensuring that the extent to which the pressure measuring tube is supported does not alter, and 'resetting' the read-out, lifting and replacing the feed arm. 2) Record the ambient temperature and pressure. 3) Shut off the supply valve.

Calculation of Results : 1) Convert the pressure reading to N/m2, and add the ambient pressure to obtain the absolute inlet stagnation pressure P0 and its ratio to the ambient. The stagnation temperature T0 is just the ambient temperature, because the heat of compression has been lost. 2) Convert the volumetric flow rates, which are read in litres/min at Normal Temperature and Pressure (N.T.P, 288 K and 1.013 bar), to m3/s by making the obvious change of units and then dividing by P0TNTP . PNTP T0

This is necessary because the rotameter actually measures the dynamic

head, which is proportional to (volumetric flow rate)2 x density. Convert volumetric flow rates, V , to mass flow rates, m , by multiplying by the density, P RT , where 0

0

the gas constant R=287 J/kg /K. 3) Subtract the zero-flow weights from the readings with flow and convert these measurements of thrust to N, using g=9.81 m/s2. 4) Calculate the nominal nozzle exhaust area, At. Analysis : 1) Calculate the value of Q =

m T0

for each of the conditions and plot it against the At P0 pressure ratio. At what pressure ratio is there an indication that the flow is choked? Do these values correspond with theory for a fluid with γ = 1.4 , to within the experimental error? (Assess the independent errors in the constituent parameters and combine them.) 2) Deduce the Mach number at the exhaust plane for each of the conditions, and plot it as a function of the pressure ratio. 3) Plot the non-dimensional thrust, F/At/Pa , against the pressure ratio and compare it with the theoretical curves for a choked nozzle and for an isentropic expansion. Again, assess the errors. Report : Describe, in your own words, the apparatus and the procedure you followed [1 mark], and the way in which the results were calculated [2 marks]. Also, quote the relevant theory [2 marks]. Discuss the trends in your results and the possible reasons for the discrepancy between them and the theory, particularly in relation to the uncertainties in your data, and the extent to which the theory is applicable [4 marks]. The overall layout of the report is important [1 mark]. EA, FM (CJL) 2014