T.G. Karayiannis

Flow Patterns and Heat Transfer for Flow Boiling in Small to Micro Diameter Tubes

An overview of the recent developments in the study of flow patterns and boiling heat transfer in small to micro diameter tubes is presented. The latest results of a long-term study of flow boiling of R134a in five vertical stainless-steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1, and 0.52 mm are then discussed. During these experiments, the mass flux was varied from 100 to 700 kg/m2s and the heat flux from as low as 1.6 to 135 kW/m2. Five different pressures were studied, namely, 6, 8, 10, 12, and 14 bar. The flow regimes were observed at a glass section located directly at the exit of the heated test section. The range of diameters was chosen to investigate thresholds for macro, small, or micro tube characteristics. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapor quality for low quality values. At higher quality, the heat transfer coefficients decreased with increasing quality, indicating local transient dry-out, instead of increasing as expected in macro tubes. There was no significant difference between the characteristics and magnitude of the heat transfer coefficients in the 4.26 mm and 2.88 mm tubes but the coefficients in the 2.01 and 1.1 mm tubes were higher. Confined bubble flow was first observed in the 2.01 mm tube, which suggests that this size might be considered as a critical diameter to distinguish small from macro tubes. Further differences have now been observed in the 0.52 mm tube: A transitional wavy flow appeared over a significant range of quality/heat flux and dispersed flow was not observed. The heat transfer characteristics were also different from those in the larger tubes. The data fell into two groups that exhibited different influences of heat flux below and above a heat flux threshold. These differences, in both flow patterns and heat transfer, indicate a possible second change from small to micro behavior at diameters less than 1 mm for R134a.

Pool boiling on modified surfaces using R-123

Saturated pool boiling of R-123 was investigated for five horizontal copper surfaces modified by different treatments, namely, an emery-polished surface, a fine sandblasted surface, a rough sandblasted surface, an electron beam-enhanced surface, and a sintered surface. Each 40-mm-diameter heating surface formed the upper face of an oxygen-free copper block, electrically heated by embedded cartridge heaters. The experiments were performed from the natural convection regime through nucleate boiling up to the critical heat flux, with both increasing and decreasing heat flux, at 1.01 bar, and additionally at 2 bar and 4 bar for the emery-polished surface. Significant enhancement of heat transfer with increasing surface modification was demonstrated, particularly for the electron beam-enhanced and sintered surfaces. The emery-polished and sandblasted surface results are compared with nucleate boiling correlations and other published data.

R134a flow patterns in small diameter tubes

Abstract R134a vapour-liquid two-phase flow patterns were studied in vertical small-diameter tubes. The observed flow patterns include bubbly, dispersed bubble, slug, churn, annular, and mist flow. Six integrated flow pattern maps, derived for two internal diameters (2.01 and 4.26 mm) and three different pressures (6.0, 10.0, 14.0 bar), are presented. Some transition boundaries, such as slug-churn and churn-annular, were found to be very sensitive to diameter and pressure. In contrast the boundaries of dispersed bubble-churn and bubbly-slug are less affected. The transition boundaries are compared with the existing models for normal size tubes showing significant differences.

Pressure drop and heat transfer characteristics for single-phase developing flow of water in rectangular microchannels

Experiments were conducted to investigate the pressure drop and heat transfer characteristics of single-phase flow of de-ionized water in single copper microchannels of hydraulic diameters 0.438 mm, 0.561 mm and 0.635 mm. The channel length was 62 mm. The experimental conditions covered a range of mass flux from 500 to 5000 kg/m2 s in the laminar, transitional and low Reynolds number turbulent regimes. Pressure drop was measured for adiabatic flows with fluid inlet temperatures of 30°C, 60°C and 90°C. In the heat transfer tests, the heat flux ranged from 256 kW/m2 to 519 kW/m2. Friction factors and Nusselt numbers determined from the measurements were higher than for fully-developed conditions, but in reasonable agreement with predictions made using published solutions for hydrodynamically and thermally developing flow. When entrance effects, experimental uncertainties, heat losses, inlet and exit losses, thermal boundary conditions and departure from laminar flow were considered, the results indicate that equations developed for flow and heat transfer in conventional size channels are applicable for water flows in microchannels of these sizes.

An investigation of a novel cooling system for chilled food display cabinets

Abstract The modern retail cabinets that are used for chilling and displaying food in shops are described in this paper. The deficiencies of the purely convective heat transfer mechanism used to cool food in modern cabinets are highlighted. A novel heat transfer system that provides an integrated conductive/convective cooling mechanism is then proposed. A purpose-developed finite difference model and its application in the study of the novel cooling system are presented in this paper. The model was used to evaluate the performance of the mechanism compared with the conventional convective system. The results indicate that the proposed novel system can provide improved heat transfer, which contributes to lower core food temperatures of approximately 2.5−3.5 K. This can lead to significant reductions in energy and capital costs as well as improvements in food quality and shelf-life. Furthermore, the use of this cooling system could avoid the requirement for electric defrost, which is energy-intensive.

Single-phase laminar flow heat transfer from confined electron beam enhanced surfaces

An experimental investigation of the thermal-hydraulic characteristics for single-phase flow through three electron beam enhanced structures was conducted with water at mass flow rates from 0.005 kg/s to 0.045 kg/s. The structures featured copper heat transfer surfaces, approximately 28 mm wide and 32 mm long in the flow direction, with complex three-dimensional (3D) electron beam manufactured pyramid-like structures. The channel height varied depending on the height of the protrusions and the tip clearance was maintained at 0.1–0.3 mm. The average protrusion densities for the three samples S1, S2, and S3 were 13, 11, and 25 per cm2 with protrusion heights of 2.5, 2.8, and 1.6 mm, respectively. The data gathered were compared to those for a smooth channel surface operating under similar conditions. The results show an increase up to approximately three times for the average Nusselt number compared with the smooth surface. This is attributed to the surface irregularities of the enhanced surfaces, which not only increase the heat transfer area but also improve mixing, disturb the thermal and velocity boundary layers, and reduce thermal resistance. The increase in heat transfer with the enhanced surfaces was accompanied by an increase of pressure drop, which has to be considered in design.

Flow Boiling of R134a and R245fa in a 1.1 mm Diameter Tube

New refrigerants are required for cooling systems due to the fact that refrigerants like R134a are about to be phased out. This paper presents a comparison between the flow boiling heat transfer and pressure drop results of refrigerants R245fa and R134a. The experiments with R245fa were conducted in a vertical cold drawn stainless steel tube with an inner diameter of 1.1 mm and heated length of 150 mm. Experimental conditions include: mass flux range 100–400 kg/m2s, heat flux range 10–60 kW/m2, pressures of 8 and 10 bar and 1.9 and 2.5 bar for R134a and R245fa corresponding to saturated temperatures 31 °C and 39 °C and exit vapour quality range 0–0.95. The data for R134a were obtained earlier using the same experimental facility at the same experimental conditions and with the same test tube. The results demonstrated that refrigerant properties have a significant effect on heat transfer and pressure drop. The pressure drop of R245fa is higher by up to 300% compared to that of R134a at similar conditions. In addition, the effect of mass flux and heat flux on the local flow boiling heat transfer coefficient was different. Heat transfer coefficients of R245fa showed a greater dependence on vapour quality. The agreement with past heat transfer correlations is better with R134a than with R245fa.Copyright

Determining heat transfer coefficients using evolutionary algorithms

This article presents a way of determining heat transfer calibrations for multi-objective and single-objective optimization by means of genetic algorithms. The need for optimization arises from the necessity for mathematical model validation and is very relevant to practical applications. The SPEA algorithm is used for multi-objective optimization. Scalarization of the fitness function is also addressed in combination with a small population size in order to keep the computational cost of the problem to a minimum. This is because of the time spent for a fitness function evaluation which comes from numerical solution of the nonlinear partial differential equations. The result is that a value of the heat transfer coefficient is determined that produces minimal difference between experimental results and numerical predictions.

A Study of Discrepancies in Flow Boiling Results in Small to Micro Diameter Metallic Tubes

There is a disagreement in the reports on flow boiling heat transfer on the dependence of the local heat transfer coefficient on local vapour quality, mass and heat flux and system pressure. As a result, various conclusions were reported about the dominant heat transfer mechanism(s) in small to micro diameter tubes. Yet, the reasons behind this large disagreement are not clear. The current study investigated experimentally two important parameters that may contribute in explaining the scatter in the published heat transfer results. The first parameter is the tube inner surface characteristics whereas the second is the length of the heated section. The surface effect was experimentally investigated through examining two stainless steel tubes manufactured by two different methods. The first tube is a seamless cold drawn tube whilst the second is a welded tube. The two tubes have similar design and dimensions and were investigated at 8 bar system pressure and 300 kg/m2 s mass flux. The inner surface of the two tubes was examined using a scanning electron microscope (SEM) and was found to be completely different. The heat transfer results demonstrated that the trend of the local heat transfer coefficient versus local vapour quality in the seamless cold drawn tube is completely different from that in the welded tube. Three heated lengths were investigated for a seamless cold drawn tube with an inner diameter of 1.1 mm over a wide range of experimental conditions; mass flux range of 200–500 kg/m2 s, system pressure of 6–10 bar, inlet sub-cooling value of about 5K and exit quality up to about 0.95. The results indicated that the heated length strongly influences the magnitude as well as the local behaviour of the heat transfer coefficient. There is a progression from nucleate boiling to convective boiling as the heated length increases. The variation in the heat transfer coefficient due to differences in the heated length may also influence the performance of the existing micro scale heat transfer correlations. The flow patterns observed at the exit of each test section are also presented.Copyright

Selected Papers From the Third Micro and Nano Flows Conference

ABSTRACTThe 5th Micro and Nano Flows Conference was held at the Politechnico Di Milano between 11 and 14 of September 2016, see http://mnf2016.com. The MNF Conference series originated in Strathcly...