Saturated flow boiling in small- to micro- diameter metallic tubes: Experimental results and modeling

Some results of a long-term study of flow boiling patterns, heat transfer rates and pressure drop of R134a at pressures of 6-14 bar in five vertical stainless steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1 and 0.52 mm are presented in this paper. The flow regimes in the 4.26 mm to 1.1 mm tubes were identified as dispersed bubble, bubbly, slug, churn, annular and mist flows. As the diameter was reduced, progressively slimmer vapour slugs, a thinner liquid film around the vapour slug and a less chaotic vapour-liquid interface in churn flow were observed. Confined flow appeared first in the 2.01 mm tube. Dispersed bubble flow was not observed in the smallest tube (0.52 mm) for the range studied in runs in which wavy film flow occurred. 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 vapour quality for low quality values. At higher quality, the heat transfer coefficients decreased with quality, indicating local dryout. The heat transfer characteristics of the 0.52 mm tube were 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. The pressure drop and heat transfer results were compared with existing correlations but with some limited success. Recent progress on mechanistic models for heat transfer along with comparisons and recommendations are included in the paper

An experimental study of dynamic flow of nanofluid with different concentrations

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Current reported data of nanofluid concentration is almost all based on TEM observation, which is in a static situation. No data of dynamic concentration during flow is reported. In the present study, an experimental measurement based on nuclear magnetic resonance (NMR) of monitoring the dynamic concentrations of nanofluid flow is carried out. It is demonstrated that the ferrofluid with Fe3O4 as its nanoparticles coated with surfactant as a special type of nanofluid can be used as T2 contrast agent in NMR scanning as well as a magnetic and thermal sensitive nanoparticle solution that would enhance heat transfer

Design Configurations and Operating Limitations of an Oscillating Heat Pipe

Passive and compact heat dissipation systems are and will remain vital for the successful operation of modern electronic systems. Oscillating heat pipes (OHPs) have been a part of this research area since their inception due to their ability to passively manage high heat fluxes. In the current investigation, different designs of tubular, flat plate, and multiple layer oscillating heat pipes are studied by using different operating parameters to investigate the operating limitations of each design. Furthermore, selective laser melting was demonstrated as a new OHP manufacturing technique and was used to create a compact multiple layer flat plate OHP. A 7-turn tubular oscillating heat pipe (T-OHP) was created and tested experimentally with three working fluids (water, acetone, and n-pentane) and different orientations (horizontal, vertical top heating, and vertical bottom heating). For vertical, T-OHP was tested with the condenser at 0°, 45° and 90° bend angle from the y-axis (achieved by bending the OHP in the adiabatic) in both bottom and top heating modes. The results show that T-OHP thermal performance depends on the bend angle, working fluid, and orientation. Another design of L-shape closed loop square microchannel (750 x 750 microns) copper heat pipe was fabricated from copper to create a thermal connector with thermal resistance \u3c 0.09 ˚C/W for electronic boards. The TC-OHP was able to manage heat rates up to 250 W. A laser powder bed fusion (L-PBF) additive manufacturing (AM) method was employed for fabricating a multi-layered, Ti-6Al-4V oscillating heat pipe (ML-OHP). The 50.8 x 38.1 x 15.75 mm3 ML-OHP consisted of four inter-connected layers of circular mini-channels, as well an integrated, hermetic-grade fill port. A series of experiments were conducted to characterize the ML-OHP thermal performance by varying power input (up to 50 W), working fluid (water, acetone, NovecTM 7200, and n-pentane), and operating orientation (vertical bottom-heating, horizontal, and vertical top-heating). The ML-OHP was found to operate effectively for all working fluids and orientations investigated, demonstrating that the OHP can function in a multi-layered form, and further indicating that one can ‘stack’ multiple, interconnected OHPs within flat media for increased thermal management

An Experimental and Analytical Study on Pulsating Heat Pipe

With the rapid development of the semiconductor material technology, the operating power of processors has the trend to increase higher. The pulsating heat pipe (PHP) or oscillating heat pipe with the high performance and simple structure is a promising heat transfer device for a lot of applications in further technology. In the present study, a three-dimensional closed-loop pulsating heat pipe has been simulated and compared with the experiment. The present study concentrates on analyzing and predicting the behavior of motion of fluid flow inside the pulsating heat pipe. A model of pulsating heat pipe with eight-turn was fabricated using Pyrex tubes with the inner diameter of 1.85 mm. The boundary temperatures for the evaporator and condenser were 80ºC and 25ºC. For working fluid, R123 was employed in this study because of its sensibility characteristic on the motion. The charging ratios were 50% and 60%. Flow visualization through the transparent chambers using a high-speed camera was recorded to study the flow motion. The circulation motion was observed in both experiment and simulation. The simulation results of the motion characteristics showed a good agreement with the experimental data. The simulation results of flow pattern, heat transfer rate, and pressure were also discussed. The results have been analyzed to understand better about the mechanism of PHP and they provided lessons for progressing to further modeling.Abstract iii List of Tables vii List of Figures viii Chapter 1 Introduction 1 1.1 Fundamental definition of PHP 1 1.2 Applications of PHP 3 Chapter 2 Literature Review 4 2.1 Experimental work 4 2.1.1 Influence of geometric parameters 5 2.1.2 Influence of physical properties of working fluid 7 2.1.3 Influence of operational parameters 10 2.2 Analytical work 14 Chapter 3 Experiment 16 3.1 Introduction 16 3.2 Experimental apparatus 16 3.2.1 Experimental apparatus and procedure 16 3.2.2 Measurement 20 3.3 Results and discussions 22 3.3.1 Visual observation of flow patterns 22 3.3.2 Heat transfer rate 24 3.3.3 Wall temperature and pulsating frequency 26 3.3.4 Summary of experiment 34 Chapter 4 Numerical Analysis 35 4.1 Mathematical models 35 4.1.1 Governing equations 35 4.1.2 Phase change model 37 4.1.3 Heat transfer model 38 4.2 Geometry and mesh 38 4.2.1 Geometry model 38 4.2.2 Meshing model 39 4.3 Initial and boundary conditions 41 4.4 Solution procedure 44 4.5 Results and discussions 46 4.5.1 Circulating flow in PHP 46 4.5.2 Wall temperature 51 4.5.3 Heat transfer rate 55 4.5.4 Pressure data 59 Chapter 5 Conclusion 62Maste

Multifactoring concept – A key to investigation of forced-eoiling in microsystems

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.In the present paper forced-boiling in microsystems is considered in the light of fundamentals of boiling heat transfer such as local temperature pulsations of heating surface (Moore and Mesler, 1961), pumping effect of growing bubble (PEGB)(Shekriladze, 1966), a model of “the theatre of director” (MTD) (Shekriladze and Ratiani, 1966) and multifactoring concept (MFC) (Shekriladze, 2006). An attempt is made to resolve a contradiction between accordance of heat transfer process to developed boiling heat transfer law in the major part of experiments and qualitatively differing trends in the other part of processes. The problem of interpretation of generation of strong reverse vapor flows, related cyclical oscillations and flow instabilities also is touched. According to presented analysis leading role in specific thermo-hydrodynamic characteristics of boiling microsystems is played by so-called duration-dependent multifactoring which, by its part, is linked to transition to prolonged action of microlayer evaporation (MLE) and PEGB. As a result drastically increases a number of influencing heat transfer factors extremely complicating description of the process. At the same time prolongation of intensive stage of acting of MLE and PEGB creates prerequisites for specific thermo-hydrodynamic appearances

Design and operation of a Tesla-type valve for pulsating heat pipes

AbstractA new Tesla-type valve is successfully designed for promoting circulation in a pulsating heat pipe (PHP) and improving the thermal resistance. Its functionality and diodicity is tested by laminar single-phase modelling and by steady two-phase flow experiments. The valve is symmetrically integrated in a single-turn PHP, which reduces variabilities to give a more thorough understanding of the behaviour in PHPs. Two transparent bottom-heated PHPs, one with and one without valves, are manufactured and the flow behaviour and thermal performance is studied. The valves produced a diodicity which lead to a difference in velocity of 25% for the different flow directions. Furthermore, a decrease of 14% in thermal resistance was observed due to the addition of the valves

Experimental analysis and transient numerical simulation of a large diameter pulsating heat pipe in microgravity conditions

A multi-parametric transient numerical simulation of the start-up of a large diameter Pulsating Heat Pipe (PHP) specially designed for future experiments on the International Space Station (ISS) are compared to the results obtained during a parabolic flight campaign supported by the European Space Agency. Since the channel diameter is larger than the capillary limit in normal gravity, such a device behaves as a loop thermosyphon on ground and as a PHP in weightless conditions; therefore, the microgravity environment is mandatory for pulsating mode. Because of a short duration of microgravity during a parabolic flight, the data concerns only the transient start-up behavior of the device. One of the most comprehensive models in the literature, namely the in-house 1-D transient code CASCO (French acronym for Code Avancé de Simulation du Caloduc Oscillant: Advanced PHP Simulation Code in English), has been configured in terms of geometry, topology, material properties and thermal boundary conditions to model the experimental device. The comparison between numerical and experimental results is performed simultaneously on the temporal evolution of multiple parameters: tube wall temperature, pressure and, wherever possible, velocity of liquid plugs, their length and temperature distribution within them. The simulation results agree with the experiment for different input powers. Temperatures are predicted with a maximum deviation of 7%. Pressure variation trend is qualitatively captured as well as the liquid plug velocity, length and temperature distribution. The model also shows the ability of capturing the instant when the fluid pressure begins to oscillate after the heat load is supplied, which is a fundamental information for the correct design of the engineering model that will be tested on the ISS. We also reveal the existence of strong liquid temperature gradients near the ends of liquid plugs both experimentally and by simulation. Finally, a theoretical prediction of the stable functioning of a large diameter PHP in microgravity is given. Results show that the system provided with an input power of 185W should be able to reach the steady state after 1min and maintain a stable operation from then on