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Enhancement of single- and two- phase heat transfer: inside heat generators
Full text linkHeat generators are actually the most used system for producing domestic heat and hot water. These systems are most commonly fire tube heat generators, which consist in a shell-and-tube heat exchanger with the flue gases produced by a stationary combustion process flowing inside the tubes, and the secondary fluid, i.e. water, located in the shell.
The first Chapter of this Thesis presents an experimental and theoretical analysis of the working conditions of a three pass fire tube heat generator, while operating both in stationary and transient regimes. In the literature, at the best of the author’s knowledge, very limited research is published on such systems.
Experimental tests were performed varying heat generator working conditions as well as inserting or not turbulence generator inserts inside the tube composing the last flue gases pass.
A dynamic MatLab/Simulink model for the fire tube heat generator is presented. This has been validated using the new experimental dataset. The model is characterized by a subsystem structure which makes it easily adaptable to different geometries, and it can be used to predict the behavior of heat generators working in stationary and transient conditions.
Heat generators performances can be increased by enhancing heat transfer between the flue gases and the water inserting turbulence generators in the tube where the combustion products flow. In particular, inserting turbulators inside the last pass of fire tube heat generators leads to an increase of the system efficiency, due to reduced flue gases exit temperature which implies reduced thermal losses at the chimney. Thus, single phase heat transfer enhancement by means of turbulence generator inserts is an important field of research for the heat transfer industry.
However, it must be kept in mind that the convective heat transfer coefficient enhancement due to the turbulators presence is associated with an augmentation of the frictional losses in the system, thus both factors have to be taken into account when evaluating inserts performances.
In the second part of the present Thesis, after a review of the most common solutions presented in the literature, performance enhancement of turbulators geometries actually used by the heat generators manufacturers is evaluated by means of CFD simulations. Effects of several geometrical parameters, such as turbulator position inside the tube and pipe diameter, are analyzed with the simulations. Equations for predicting the inserts working conditions are presented. Finally, a modified geometry is presented with the aim of proposing a solution with enhanced thermal and frictional characteristics.
By recovering the latent heat in the exhaust products coming out from the heat generator it is possible to achieve extremely high system efficiencies. Thus, two-phase heat transfer is a field of interest in the heat generators industry.
During pure steam condensation the mayor resistance to the thermal transport is associated to the condensate layer which forms at the wall. Reducing, or eventually removing, the liquid film thickness at the wall will thus lead to an enhancement of the condensation heat transfer coefficient, increasing the heat transfer per unit area. This means the possibility to get higher performances of the system maintaining the same geometry, or to reduce the heat transfer area (thus the system cost) maintaining the same net effect.
The last Section of the present Thesis is focused on the enhancement of the condensation heat transfer coefficient by means of nano-engineered surfaces. Effect of surface wetting properties on the condensation process and performance is analyzed by studying the behavior of conventional, superhydrophilic, hydrophobic and superhydrophobic surfaces during pure steam flow condensation at different mass velocities. The aim of the research is to remove (dropwise condensation mode) or eventually reduce (filmwise condensation mode) the condensate layer over the wall during the two-phase process, by acting on surface superficial characteristics, as well as to evaluate the effect of surface roughness on the filmwise condensation heat transfer coefficient
Experimental analysis of steam condensation over conventional and superhydrophilic vertical surfaces
No full textFrictional Pressure Drop during Two-Phase Flow of Pure Fluids and Mixtures in Small Diameter Channels
No full textTwo-phase flow is widely encountered in minichannels heat exchangers such as air-cooled condensers and evaporators for automotive, compact devices for electronic cooling and aluminum condenser for air-conditioning applications. In the present work, frictional pressure drop during adiabatic liquid-vapor flow is experimentally investigated inside a single 0.96 mm diameter minichannel. Tests have been run with three mixtures of R32/R1234ze(E) (23/77%, 50/50% and 75/25% by mass composition) at mass flux ranging between 200 and 600 kg m-2 s-1. Since pressure drop has a strong influence on the two-phase heat transfer, it is crucial to have reliable pressure drop prediction methods for two-phase heat transfer modeling and optimization. Therefore, with the aim of extending its validity range, a model to calculate the frictional pressure gradient during two-phase flow in small diameter channels is tested against the present two-phase pressure drop database. An assessment is also done with two low-GWP refrigerants: the halogenated olefin R1234ze(E) and the hydrocarbon R290. The present model accounts for the effect of internal surface roughness as a function of the liquid-only Reynolds number
A prediction method of frictional pressure drop during two-phase flow in small diameter channels
No full textTwo-phase flow in minichannels is widely present in evaporators and condensers, such as, for example, air-cooled condensers for automotive and air-conditioning applications. It is crucial to have reliable pressure drop prediction methods for two-phase heat transfer modeling and optimization because pressure drop have a strong influence on the two-phase heat transfer. In this work, a model to calculate the frictional pressure gradient during two-phase flow in small diameter channels is presented: it accounts for the effects of surface roughness on the internal surface. Predictions by the present model are compared against a new two-phase pressure drop database to enlarge its validity range. The new pressure drop database is obtained from experimental tests with refrigerants R290 (propane), R1234ze(E), and two mixtures R32/R1234ze(E) (23/77% and 50/50% by mass composition) at mass flux ranging between 200 and 800 kg m-2 s-1
Experiments and updated model for two phase frictional pressure drop inside minichannels
No full textDropwise condensation on superhydrophobic nanostructured surfaces: literature review and experimental analysis
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