Heat Transfer Correlation Development

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Introduction

Throughout the years extensive studies were performed on fluids flowing within tubes. These studies started as far back as 1883 when Osborne Reynolds introduced a dye flowing in water to distinguish between two distinct regimes he called “direct” and “sinuous” (Reynolds, 1883) or, in modern terms, laminar and turbulent regimes. It was this groundbreaking work which led other researchers to pursue and demystify the true nature of these flow regimes. In 1839 and 1840, Hagen and Poiseuille, respectively, studied hydrodynamically fully developed viscous/laminar isothermal flows within tubes (White, 1991). They showed that the pressure drop within a tube is directly proportional to the shear stress at the tube wall and inversely proportional to the diameter of the tube. This shear stress is non-dimensionalised with respect to the dynamic pressure to obtain a friction factor, one known as the Fanning friction factor and the other the Darcy friction factor. These friction factors are widely used in the design of piping systems as well as heat exchangers to determine the pumping power consumption required for the system.

Work of Ghajar and co-workers

In this chapter it has already been highlighted that Ghajar and co-workers have done a substantial amount of work on different types of inlets for smooth tubes. In Chapter 2 more detail is given of all the work that they have done. To summarise, they have investigated the heat transfer and pressure drop characteristics for different types of inlets. In their work they used (1) constant heat flux heating inside a (2) smooth tube only, investigating only a (3) single diameter (15.84 mm) tube, (4) measured local heat transfer and pressure drop data and (5) used a combination of water and glycol as their working fluid.

Layout of the Thesis

The thesis starts off with a look into the state of the art regarding transitional flow. This forms part of the literature survey in Chapter 2. Next, the experimental system is discussed in Chapter 3. Included in this section is the method for calculating friction factors and heat transfer coefficients. The results of the uncertainty analysis will be shown, with the validation of the system with regard to these methods also being shown. The results will be discussed in a span of four chapters. The chapters are separated in terms of smooth and enhanced tubes and the different types of inlets. Thus, Chapter 4 will discuss the fully developed smooth tube results, while Chapter 5 will discuss the smooth tube results with regards to the various inlet profiles. Chapter 6 will then contain the results of the fully developed enhanced tubes, while Chapter 7 will contain the results for the enhanced tubes with the various inlet profiles.

State of the Art

It is accepted in literature that transition from laminar to turbulent flow inside tubes occurs at a Reynolds number of approximately 2 300. Although this is an accepted value, transition in reality occurs in the range of Reynolds numbers between 2 300 and 10 000 (Tam and Ghajar, 1997). It is normally advised when designing heat exchangers to remain outside these limits due to the uncertainty and flow instability of this region. Large pressure variations are also encountered in this region since the pressure gradient required to move the fluid in laminar and turbulent flow could vary by an order of magnitude.

1 Introduction
1.1 Introduction
1.2 Objectives
1.3 Work of Ghajar and co-workers
1.4 Layout of the Thesis
1.4.1 Notation
2 Literature Survey
2.1 State of the Art
2.2 Correlations
2.2.1 Heat Transfer
2.2.2 Friction Factors
2.2.3 Enhanced Tubes
2.3 Conclusion
3 Experimental Set-up, Data Analysis and Validation
3.1 Introduction
3.2 Experimental Set-up
3.2.1 Calming Section
3.2.2 Test Section
3.3 Data Reduction
3.3.1 Heat Transfer Coefficient
3.3.2 Friction Factor
3.4 Instruments
3.4.1 Thermocouples
3.4.2 Pressure Drop
3.4.3 Flow Meters
3.5 Uncertainties
3.6 Experimental Procedure
3.7 Validation
3.7.1 Heat Transfer Coefficients
3.7.2 Friction Factor
3.8 Conclusion
4 Results: Fully Developed Smooth Tube
4.1 Introduction
4.2 Adiabatic Friction Factors
4.2.1 Correlation
4.3 Heat Transfer
4.3.1 Diabatic Friction Factors
4.3.2 Correlation
4.4 Conclusion
5 Results: Entrance Effects for Smooth Tubes
5.1 Introduction
5.2 Adiabatic Friction Factors
5.2.1 Correlation
5.3 Heat Transfer
5.3.1 Diabatic Friction Factors
5.3.2 Correlation
5.4 Conclusion
6 Results: Fully Developed Enhanced Tubes
6.1 Introduction
6.2 Adiabatic Friction Factors
6.2.1 Correlation comparison
6.2.2 Adiabatic Friction Factor Correlation
6.3 Heat Transfer
6.3.1 Diabatic Friction Factors
6.3.2 Correlation comparison
6.3.3 Heat Transfer Correlation Development
6.3.4 Diabatic Friction Factor Correlation Development
6.4 Performance Evaluation
6.5 Conclusion
7 Results: Entrance Effects for Enhanced Tubes
7.1 Introduction
7.2 Adiabatic Friction Factors
7.2.1 Correlation
7.3 Heat Transfer
7.4 Diabatic Friction Factors
7.5 Conclusion
8 Conclusion
8.1 Summary
8.2 Results
8.2.1 Adiabatic Friction Factors
8.2.2 Heat Transfer
8.2.3 Correlations
8.3 Conclusion
8.4 Future Work

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SINGLE-PHASE HEAT TRANSFER AND PRESSURE DROP OF WATER COOLED AT A CONSTANT WALL TEMPERATURE INSIDE HORIZONTAL CIRCULAR SMOOTH AND ENHANCED TUBES WITH DIFFERENT INLET CONFIGURATIONS IN THE TRANSITIONAL FLOW REGIME