Browsing by Author "Green, WJ"
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- ItemAdaptation of a Freon-12 CHF correlation to apply for water in uniformly heated vertical tubes, Part 2 - based on CHF data for water pressures in the range 6-20 MPa(Australian Atomic Energy Commission, 1982-03) Green, WJAn examination of more than 5000 sets of experimental data for critical heat flux (CHF) in uniformly heated vertical tubes internally cooled by high pressure water has shown that the CHF correlation proposed in Part 1 of this work is accurate for water at pressures up to approximately 17 MPa provided that minor modifications are made to the Prandtl number index and the saturation boiling length function. For pressures greater than 17 MPa CHF values calculated from the correlation are increasingly lower than the experimental data particularly at low saturation boiling length ratios (<100). This deficiency results from the significance of the surface tension number in the correlation and the accuracy to which the values of surface tension and latent heat are known as they approach zero at the critical pressure. As in the previous work data have been excluded in which either the mass flux is less than 300 kg s -1m -2 or thermal equilibrium exit qualities are less than 0.1.
- ItemAdaptation of a Freon-12 critical heat flux correlation to correlate water data from uniformly heated vertical tubes, Part I - based on critical heat flux data for water at pressures of 3 to 14 MPa(Australian Atomic Energy Commission, 1981-12) Green, WJComparisons have been made between experimental critical heat flux (CHF) data for upflow of water in uniformly heated vertical tubes and values calculated from an empirical CHF correlation developed from Freon-12 data. When this correlation is re-evaluated to account for vapour Prandtl number effects very good agreement is obtained between experimental data and calculated values over a wide range of coolant conditions. Comparison of values calculated from the revised correlation with 2063 sets of CHF data obtained from experiments with water in vertical uniformly heated tubes shows a mean ratio of the calculated to experimental CHF of 0.82 and an r.m.s. error of 5.8 per cent for the following coolant conditions: (1) local pressure of 3.4 to 12 MPa; (2) mass flux greater than approx. 300 kg s-1m - 2 and (3) thermal equilibrium value of exit quality greater than 0.1.
- ItemAn experimental and analytical study of transient heat transfer in the region of dryout for a heated tube using Freon-12 as coolant.(Australian Atomic Energy Commission, 1978-11) Green, WJPower transient experiments have been performed using vertical round tube test sections to determine the heat transfer characteristics associated with a change from pre-dryout to post-dryout flow boiling conditions. The test sections were heated by passing electric current along the tube wall, and cooled internally by Freon-12 flowing upwards through the tube. Five steel tubes of various sizes were used (internal diameters in the range 7.1-21.3 mm, wall thicknesses 1.1-2.0 mm, and lengths 2.86-3.94 m). Heat transfer results are reported for coolant mass fluxes in the range 270-3270 kg m"2 s~', at a pressure of 1.04 MPa, with exit qualities in the range 0,3-1.0. The transients were initiated by small step changes, corresponding to increases of a few per cent, in power input to the test section, after initial steady-state conditions had been established with heat fluxes just below the corresponding critical heat flux value. The heat transfer characteristics associated with the transients were determined from measurements of the wall temperature transients. A computer code (THETRAN) was developed for analysing the experimental heat transfer results by combining established heat transfer codes. The new code takes account of both transient thermal diffusion within the tube wall and transient thermal energy transfer to the coolant. The analysis involved predicting wall temperature variations as a function of time and comparing these predictions with the corresponding temperature traces recorded in the experiments. The sensitivity of this technique was investigated in relation to the various relevant input parameters, including, for example, changes in the magnitude of predicted critical heat flux. Analysis of the experimental results has shown that post-dryout heat transfer characteristics can be considered in terms of four regions. In order of increasing surface temperature from the onset of boiling crisis conditions, these are: (i) a transition boiling region in which surface heat flux decreases (from the critical heat flux value) with increase in surface temperature, (ii) a minimum heat flux film boiling region, in which the heat flux may be constant over a substantial range of surface temperature values, (iii) an intermediate film boiling region, and (iv) a high temperature film boiling region. In relation to the temperature response of the tube wall under these transient conditions the transition boiling region had only a slight effect; the minimum heat flux film boiling region had a very significant effect and sometimes remained while substantial increases in surface temperature occurred. (Examination of the data obtained for this region indicated that this minimum heat flux is flow-dependent in a relationship of the form qmm (Re) where n ~ 0.5.); and the intermediate and high temperature film boiling regions also had significant effects on the temperature response. (Published film boiling correlations have been slightly modified and then utilised for correlating the data from these regions.) Critical heat flux (CHF) values deduced from experimental data are shown to be in reasonable agreement with a modified Groeneveld CHF correlation.
- ItemInvestigation of critical heat fluxes in vertical tubes internally cooled by Freon-12, Part II - the development of a critical heat flux correlation for uniformly heated tubes(Australian Atomic Energy Commission, 1981-12) Green, WJBased on the parameter groups derived from a dimensional analysis a correlation for predicting critical heat fluxes (CHF) in a uniformly heated tube cooled internally by Freon-12 has been developed by systematically examining experimental data to establish the interrelationship of these dimensionless groups. Although the basic correlation agrees with much experimental data it is inaccurate for short tubes or low mass fluxes. To compensate for these effects corrective terms were introduced to provide a more generalised form. The correlation has been tested against approximately 2000 experimental data. It was found that apart from data at low exit qualities (less than 0.1) or short boiling lengths less than approximately 50 the correlation agrees with the data to within +- 10 per cent for boiling length ratios less than approximately 150; for values greater than 150 more than 90 per cent of points agree with the correlation to within +- 5 per cent.
- ItemInvestigation of critical heat fluxes in vertical tubes internally cooled by Freon-12, Part I - critical heat flux experiments with axially uniform and non-uniform heating and comparisons of data with selected correlations.(Australian Atomic Energy Commission, 1981-08) Green, WJ; Stevens, JRExperiments have been performed using vertical heated tubes cooled internally by Freon-12 to determine critical heat fluxes (CHFs) for both a uniformly heated section and an exit region with a separately controlled power supply. Heated lengths of the main separately were 2870 mm (8.48 and 16.76 mm tube bores) and 3700 mm (for 21.34 mm tube bore); heated length of the exit section was 230 mm. Coolant pressures exit qualities and mass fluxes were in the range 0.9 to 1.3 MPa 0.19 to 0.86 and 380 to 2800 kg m -2 -1 respectively. The data have been compared with published empirical correlations specifically formulated to predict CHFs in Freon-cooled vertical tubes; relevant published CHF data have also been compared with these correlations. These comparisons show that even over the ranges of conditions for which the correlations were developed predicted values are only accurate to within +-20 per cent. Moreover as mass fluxes increase above 3500 kg m - 2 -1 the modified Groeneveld correlation becomes increasingly inadequate and the Bertoletti and modified Bertoletti correlations under-predict CHF values by increasing amounts. At mass fluxes below 750 kg m - 2 -1 the Bertoletti correlations exhibit increasing inaccuracy with a decrease in mass flux. For non-uniform heating the correlations are at variance with the experimental data.
- ItemPredicted HIFAR fuel element temperatures for postulated loss-of-coolant accidents(Australian Atomic, 1987-04) Green, WJA two-dimensional theoretical heat transfer model of a HIFAR Mark IV/Va fuel element has been developed and validated by comparing predicted thermal performances with experimental temperature responses obtained from irradiated fuel elements during simulated accident conditions. Full details of the model's development and its verification have been reported elsewhere. In this report the model has been further used to ascertain acceptable limits of fuel element decay power for the start of two specific LOCAs which have been identified by the Regulatory Bureau of the AAEC. For a single fuel element which is positioned within a fuel load/ unload flask and is not subjected to any forced convective air cooling the model indicates that fission product decay powers must not exceed 1.86 kW if fuel surface temperatures are not to exceed 450 deg C. In the case of a HIFAR core LOCA in which the complete inventory of heavy water is lost it is calculated that the maximum fission product decay power of a central element must not exceed 1.1 kW if fuel surface temperatures are not to exceed 450 deg C anywhere in the core.
- ItemA sensitivity study of the heat transfer processes involved in a hot-leg rupture in a pressurised water-cooled system(Australian Atomic Energy Commission, 1977-09) Green, WJSeries of computer experiments was performed, using the reactor thermal analysis code THETAl-B, to ascertain the significance and relative importance of the various heat transfer processes in relation to the prediction of maximum surface temperatures for: (i) the fuel cladding during the blowdown phase of a loss of coolant accident (LOCA) for a hot-leg rupture in a pressurised water reactor system; and (ii) an electrically heated tube of a simple water-cooled test assembly, subjected to a corresponding hot-leg blowdown, for which experimental results have been published. The significance of several factors was considered, including the choice of heat transfer correlation for a particular heat transfer regime, and inlet coolant flow conditions. For both cases the predicted maximum surface temperature was sensitive to a number of factors, including choice of heat transfer correlations for (i) critical heat flux, (ii) flow boiling transition region and, in particular, (iii) stable film boiling.
- ItemTheoretical models to predict the transient heat transfer performance of HIFAR fuel elements under non-forced convective conditions(Australian Atomic Energy Commission, 1987-04) Green, WJSimple theoretical models have been developed which are suitable for predicting the thermal responses of irradiated research fuel elements of markedly different geometries when they are subjected to loss-of-coolant accident conditions. These models have been used to calculate temperature responses corresponding to various non-forced convective conditions. Comparisons between experimentally observed temperatures and calculated values have shown that a suitable value for surface thermal emissivity is 0.35; modelling of the fuel element beyond the region of the fuel plate needs to be included since these areas account for approximately 25 per cent of the thermal power dissipated; general agreement between calculated and experimental temperatures for both transient and steady-state conditions is good - the maximum discrepancy between calculated and experimental temperatures for a HIFAR Mark IV/V fuel element is approx 70 deg C and for an Oak Ridge Reactor (ORR) box-type fuel element approx 30 deg C; and axial power distribution does not significantly affect thermal responses for the conditions investigated. Overall the comparisons have shown that the models evolved can reproduce experimental data to a level of accuracy that provides confidence in the modelling technique and the postulated heat dissipation mechanisms and that these models can be used to predict thermal responses of fuel elements in accident conditions that are not easily investigated experimentally.
- ItemTHETRAN - a two-dimensional heat transport code for analysis of power transients(Australian Atomic Energy Commission, 1981-04) Green, WJ; Jacobs, WSVA computer code THETRAN has been developed for two-dimensional analysis of heat transfer transients by utilising parts of two available transient heat transfer codes THETA1-B and HEATRAN. Combination of the one-dimensional coolant energy and heat transfer equations used in THETA1-B with the two-dimensional transient thermal diffusion code HEATRAN was achieved by matching two parameters (surface temperature and heat flux) at the solid/fluid interface. The resultant code is capable of analysing the transient thermal performance of a tube or duct internally or externally cooled by a flow of liquid coolant which may boil. The code was developed to analyse experimental conditions in which an electrically heated tube is subjected to a rapid change in power input while heat is removed by an internal flow of coolant at a constant mass flowrate.