HWA MEASUREMENT OF TURBULENT DIFFUSION OF A SCALAR QUANTITY

The paper deals with simultaneous measurement of the velocity and the scalar quantity by means of hot-wire anemometry. Statistical moments of the scalar quantity fluctuations can be obtained employing a dual hot-wire probe. An evaluation procedure of the quantity mean values and fluctuations is described. Results from the molar concentration measurement in binary-gas mixture are shown.


INTRODUCTION
Abstract: The paper deals with simultaneous measurement of the velocity and the scalar quantity by means of hot-wire anemometry.Statistical moments of the scalar quantity fluctuations can be obtained employing a dual hot-wire probe.An evaluation procedure of the quantity mean values and fluctuations is described.Results from the molar concentration measurement in binary-gas mixture are shown.
Hot-wire anemometry is suitable method for simultaneous measurement of the velocity and the scalar quantity of the flow.A probe with two sensors is employed in such measurements.Statistical moments of the scalar quantity, e.g. the fluid temperature T a or the molar concentration C of binary-gas mixture, can be obtained.

THEORETICAL BACKGROUND
A dual hot-wire probe consists of two sensors (subscript j=1,2).We assume the same velocity and scalar quantity at measurement "point" -on both sensors.
Relation (1) in dimensional form written for both sensors creates a system of two equations: Non-linear functions f and g depends on used type of cooling law; parameters result from calibration of the sensors.Solving the system numerically we obtain instantaneous values of observed quantities.
To reduce computational demands there is useful to separate evaluation of mean values and fluctuations.At each probe location we compute mean values Then we compute fluctuations by following relations.Sensitivities to the velocity u and the scalar quantity \ can be computed from the calibration: Anemometer output voltage E can be written as follows: A system of two equations ( 5) written for both sensors leads into: The system is badly conditioned if

S S S S
, what can be seen from eq. ( 6).Correct setup of simultaneous measurement therefore requires different ratios of sensitivities for both sensors: This can be achieved by wise choice of experimental setup.Sensitivities to the observed quantities depend on the geometry and temperature of the sensor (d w , l w , T w ).One should also pay attention to orientation of sensors and directional characteristics of the probe because of requirement of equality of effective velocities for both sensors.

CONCENTRATION MEASUREMENT
Procedure described above was applied on simultaneous measurement of the velocity and the molar concentration in binary-gas mixture.The mean values and the variances of the concentration were evaluated.A case of polynomial shaped 2D hill with the line source of tracer gas was studied.Shape of the hill was taken from Almeida at al.  Profiles of the molar concentration over the hill were calculated using described procedure.The mean values show Graph 2 and the variances show Graph 3.

CONCLUSION
Statistical moments of the velocity and the scalar quantity fluctuations can be obtained from CTA hot-wire measurement.A procedure of simultaneous measurement of the velocity and the scalar quantity with dual hot-wire probe is described.Application of the procedure to a velocity-concentration case employing two parallel wires works satisfactory.Distributions of mean values and variances of the molar concentration over a two dimensional polynomial-shaped hill is shown in the paper.
[5].Cross section of the channel was of a width of 0.1 m and a height of 0.25 m.The velocity of the main flow was set at 6 m.s -1 .An admixture gas CO 2 as a tracer enters the main airflow upstream the hill at x=-140 mm.Volume rate of admixture was set at 5e-4 m 3 .s - that corresponds with velocity in the slot of 5 m.s -1 .The results are presented in the positions of x=(0;10;20;30;40;50;70;90;110;130;150) mm (see Fig.1).