Downward flow in a heated annulus

Application Challenge 3-11 © copyright ERCOFTAC 2004

Overview of Tests

Over 100 individual sets of data are available, with various flow and heating rates. An overview of the test cases measured is given in Table 1.

No	$q\left[kW/m^{2}\right]$	Re	Gr*	Bo	Water T at inlet
1	3.48	2053	1.13E+08	7.72	15.19
2	3.48	4065	1.24E+08	0.83	15.92
3	3.48	6029	1.24E+08	0.21	15.93
4	3.48	6003	1.23E+08	0.22	15.87
5	10.44	1034	3.67E+08	265.9	15.81
6	10.44	2055	4.06E+08	28.6	16.67
7	10.44	3995	4.02E+08	2.89	16.57
8	10.44	6047	4.01E+08	0.70	16.56
9	10.44	5997	4.28E+08	0.78	17.13
10	17.41	2028	8.35E+08	64.2	18.58
11	17.41	4038	8.34E+08	6.06	18.57
12	17.41	6041	1.02E+08	1.95	20.57
13	17.41	6009	6.77E+08	1.21	16.66
14	24.37	2040	1.12E+08	83.75	18.19
15	24.37	1969	1.13E+08	94.92	18.22
16	24.37	1998	1.19E+08	96.59	18.75
17	24.37	4052	1.44E+08	10.81	20.66
18	24.37	4034	1.12E+08	8.13	18.21
19	24.37	6039	1.42E+08	2.71	20.5
20	24.37	6045	1.02E+08	1.81	17.31

Table 1: Summary of parameters in each test.

Temperature data is obtained from an array of ninety thermocouples, which are positioned to measure the temperature of the core, and the water temperatures at inlet and outlet. Measurements of velocity and turbulence are undertaken using Laser Doppler Anemometry.

The LDA system consists of a 4W Coherent Innova 70-4 Argon-ion laser generator, A DANTEC 60x40 transmitter connected by a 10 metre long fibre optic cable to a DANTEC 41x806 probe, a two component photomultiplier (PM) connected to two DANTEC 57N10 Burst Spectrum Analysers (BSA) and a computer-based data acquisition system.

The laser beam from the argon-ion laser generator is divided into two pairs of beams with different frequencies in the transmitter. These are used to measure the two components of the velocity. The four beams are carried to the probe by the fibre-optic cable, separated in the probe and then focussed to a point in the test section with a lens. The light scattered backwards by the particles in the seeding flow is collected by a receiver mounted inside the probe and converted to electrical signals in the photomultiplier. A 45o arrangement of the beams was chosen to measure the velocity components in the directions ±45o off the axis of the annular tube. The vertical and horizontal velocity components were calculated from the signals of the two channels. This arrangement has two advantages over the direct vertical and horizontal velocity measurements. First, as the signals from the two channels are similar in magnitude, the effect of electronic noise during processing and transmitting are similar and relatively small. Secondly, the symmetric arrangement of the beams allowed the measurements to be made much closer to the wall. The two BSA units are used for processing the signals.

Experiments were first undertaken to establish the optimum sampling time. Measurements of local mean velocity and turbulence profiles were extracted with various sampling times for isothermal flow and non-isothermal flow. For severe heating, some reversed flow is observed near the heated wall as a result of strong buoyancy influence, in which case the quality of results improves with increase of sampling time. However, problems are encountered in maintaining the experimental conditions steady for very long periods of time. It has been found that for Re>4000 the optimum sampling time was about 240 seconds. For 2000<Re<4000, it was 300 seconds; for Re<2000 and with strong influences of buoyancy it was 600 seconds.

A summary of all tests, in terms of problem definition and measured parameters, is shown below in Tables 2 and 3.

NAME	GNDPs	PDPs (problem definition parameters)	MPs (measured parameters)
See Table 1 for cases	Re, Gr, Bo	Inlet velocity and Temperature; Wall heat flux	detailed data	DOAPs	U, u, T	Nu

Table 2: Summary description of all test cases.

	MP1 U/V (m/s)	MP2 u'v (m/s) $u'v'{(m^{2}/s^{2})}$	MP3 $T{(^{0}C)}$	DOAPs
Cases in Table 1				Nu obtained from $q_{w}$ and T

Table 3: Summary description of all measured parameter.

Test Cases

Description of Experiments

The experimental conditions, in terms of non-dimensional parameters, are identified in Table 1. These can be used to determine the (uniform) heat flux and the mass flow rate.

Boundary Data

The inlet, which is positioned 1.5m above the Test Section, is sufficiently far away from the test section (23 effective diameters, where the effective diameter is defined as the difference between the inner and outer diameters of the annular gap) for the flow to become fully developed before entering the heated section of the core. A few experiments were undertaken to confirm this, comparing results including flow conditioning grids and a flow straightener to those without flow conditioning grids and a flow straightener.

The water temperature at the inlet is specified for each case in Table 1.

The walls are smooth.

Measurement Errors

The accuracy of the temperature data depends on the suitability of the sampling time and the accuracy of the thermocouples employed. A bounding estimate of the error in the temperature data is 0.5oC. This translates into an error of less than 2% on Nusselt number.

The accuracy of the velocity and turbulence data are currently unknown. It will be quantified at a later date.

Measured Data

For all cases, wall temperature is measured at distances of 0, 5, 10, 20, 30, 40, 50, 100, 150, 200, 250, 300 cm from the start of the heated section.

The water bulk outlet temperature is also available for each case.

Results for the temperature and Nusselt number are shown in the scanned images in Appendix D, and in tabular form in Appendix E.

Results for the velocity and turbulence data are available in electronic form.

Contributors: Mike Rabbitt - British Energy

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