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Thermocouple and pressure transducer calibration
Before testing took place, the thermocouples and pressure transducers were checked for accuracy. The thermocouples were calibrated in situ without any heat transfer occurring between the inner tube and the annular fluid. The heat exchanger was initially modified to a single loop by connecting the outlet of the annulus to the inlet of the inner tube with a plastic pipe. This modification allowed the water entering from the annulus inlet to circulate through the entire heat exchanger and exit through the inner tube outlet. All thermocouples were calibrated against two PT100 probes installed at the inlet of the annulus and outlet of the inner tube. The average measurement of the two PT100 probes was used in the calibration analysis. Water from a storage tank at different fixed temperatures was circulated for a long period of time through the exchanger until the temperature difference between the two PT100 probes was within ±0.05. The calibrations were done with the water circulated at temperatures between approximately 17 °C and 55 °C, and at intervals of approximately 5 °C. This range covered the minimum and maximum temperatures that were used during experimentation. The calibrations occurred once steady-state conditions were reached, which was when temperature variations of ±0.05 °C occurred for three minutes. Second-order polynomial calibration curves were created, with which measured data was conditioned during the data-processing stages. The calibration results of the thermocouples indicated that they were accurate in absolute terms to within a ±0.1 °C band. The 2.2 kPa pressure transducer was calibrated using a water column and a manometer with an accuracy of 0.0055 kPa. Twelve points from 0 to 2.2 kPa at intervals of 0.2 kPa were used in the calibration. A linear equation was created with which measured data was conditioned during the data-processing stages. The calibration results indicated that they were accurate to within ±0.01 kPa.
Analysis of convection types
A better understanding of convection types (natural, mixed and forced convection) that were present in the flow regimes is required for validation purposes and a better interpretation of the results. The Richardson number method was used to identify convection types. A flow regime map for annular flow could not be found in the literature and the peripheral heat transfer method could not be applied to annular flow. In the Richardson number method of determining convection types, both the free and forced convections (mixed convection) are to be considered when 0.1 ≤ Ri ≤ 10. For Ri > 10, the flow is treated as free convection. The pure forced convection is considered when Ri < 0.1. Figure 5.4 depicts the spread of experimental data within the three convection types and Reynolds number regimes (laminar, transition and turbulent). Figure 5.4a is for TS 1 where data for different degrees of wall temperature uniformity is presented. Similar data for TS 2, TS 3 and TS 4 is presented in Appendix B. Figure 5.4b is for τ = 0.99 where data for different test sections is presented. Similar data for τ = 0.975 and τ = 0.965 is presented in Appendix C. Since each test had different Re1 and Re2 values from the other (as will be presented in section 6.2), average Reynolds number limits have been used in Figure 5.4.
Introduction
1.1 Background
1.2 Problem statement
1.3 Objectives
1.4 Thesis layout
Literature study
2.1 Introduction
2.2 Factors that influence the heat transfer coefficient, pressure drop and Reynolds number limits of the transition flow regime
2.3 Methods for determining the convection types
2.4 Correlations
2.5 Summary
Experimental set-up and procedure
3.1 Introduction
3.2 Experimental facility
3.3 Test sections
3.4 Thermocouple and pressure transducer calibration
3.5 Experimental procedure
3.6 Summary
Reduction of the results
4.1. Introduction
4.2. Data reduction method
4.3. Local heat transfer analysis
4.4. Uncertainties
4.5. Summary
Preliminary data analysis
5.1. Introduction
5.2. The effect of the annulus inlet configuration on the Reynolds number limits of the transition regime
5.3. Identification of cut-off points of the flow regimes
5.4. Analysis of convection types
5.5. Validation
5.6. Summary
Influence of the degree of wall temperature uniformity
6.1 Introduction
6.2 Transition range limits
6.3 Heat transfer coefficients
6.4 Friction factors
6.5 Development of new correlations to account for the thermal boundary condition
6.6 Summary
Influence of annular passage dimensions
7.1 Introduction
7.2 Transition range limits
7.3 Heat transfer coefficients
7.4 Friction factors
7.5 Development of new correlations to account for the annular dimensions
7.6 Comprehensive correlations
7.7 Summary
Conclusions
8.1. Summary
8.2. Results
8.3. Correlations
8.4. Future works
