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Two-phase heat transfer inside minichannels: fundamentals and applications in refrigeration and solar technology
Full text linkThis thesis reports the results of many experimental tests conducted to gain a deeper insight on the two-phase heat transfer inside minichannels and to characterize the thermal performance of two refrigerants with low environmental impact: propane (R290) and R1234ze(E). Furthermore, some considerations on the application of the minichannel technology in refrigeration applications and solar concentrators are presented.
As pressure drops greatly affect the heat transfer in two-phase flow, the experimental investigation on frictional pressure gradient during adiabatic flow of R134a, R1234ze(E) and propane (R290) at different mass velocities and at saturation temperatures between 30°C and 50°C has been conducted in two single copper minichannels with a circular cross section and hydraulic diameters of 0.96 mm and 2 mm. The experimental points are compared with several models available in the open literature.
Heat transfer coefficients have been experimentally measured during the condensation at 40°C and during the vaporization at 31°C of R1234ze(E) and propane at different mass velocities inside a single circular cross section minichannel with an internal diameter of 0.96 mm. During the test runs, the refrigerant exchanges heat with a secondary fluid, that is distilled water, so the local heat flux is not constant along the measuring section and its accurate calculation becomes the main issue. An assessment of several predicting correlations has been presented for predicting the heat transfer coefficient both in condensation and in vaporization.
The condensation process inside minichannels depends on the relative importance of shear stress, gravity and surface tension, especially in presence of corners in the cross section shape. Nevertheless, few studies concern the effect of inclination. In this work, the effect of the channel orientation has been experimentally analyzed and discussed during the condensation of R134a and R32 at 40°C saturation temperature inside a single square cross section minichannel with a hydraulic diameter equal to 1.23 mm. Several configurations of the test section from vertical upward flow to vertical downward flow have been examined.
When considering the application of the minichannel technology in refrigeration, a general methodology to evaluate the potential heat transfer performance of refrigerants during in-tube condensation is a powerful tool to optimize the performance and the design of heat exchangers. The Performance Evaluation Criteria (PEC) named Penalty Factor for condensation (PF) and Total Temperature Penalization on the refrigerant side (TTP) are applied to rank several refrigerants starting from an experimental database collected in a single circular minichannel with internal diameter of 0.96 mm at the Two-Phase Heat Transfer Lab at the University of Padova.
In electronics, the minichannel technology has proved to be reliable and effective in removing high heat fluxes through small heat transfer areas. This feature has suggested to use minichannel-based receivers for solar concentration systems.
In this work, a parabolic trough linear solar concentrator is described and tested using two different minichannel-based receivers: a concentrating hybrid photovoltaic thermal (CPVT) receiver for the cogeneration of electrical energy and heat and a thermal receiver with a selective coating for the generation of heat in the medium temperature range. An optical modeling has been developed for the two cases in order to assess the optical efficiency and the flux distribution on the receiver. Tests with both the receivers have been performed using water in single-phase flow as working fluid in order to get a preliminary characterization of the whole system. The performance of the thermal receiver at medium temperature (up to 150°C) when two-phase heat transfer is realized inside the channels has been evaluated through a numerical model
Experimental study of a parabolic trough solar collector with flat bar-and-plate absorber during direct steam generation
No full textComprehensive experimental investigation of two-phase heat transfer and pressure drop with propane in a minichannel
No full textThe use of hydrocarbons as natural refrigerants inside small diameter channels allows charge
minimization and therefore it may be an interesting option in the refrigerating and heat pump
technology. The aim of the present experimental work is to fully characterize the thermal
performance of propane (R290) in minichannels by measuring frictional pressure drop,
condensation and flow boiling heat transfer coefficients inside a circular cross section horizontal
minichannel with an internal diameter of 0.96 mm and a rough inner surface. Measurements of
frictional pressure drop during adiabatic two-phase flow have been performed at mass velocity
ranging between 200 and 800 kg m-2 s-1. Local heat transfer coefficients have been measured during
condensation and during flow boiling in the mass velocity range from 100 to 1000 kg m-2 s-1. The
present database, including frictional pressure gradient, condensation and vaporization heat transfer
coefficients, is compared against predicting correlations available in the open literature
Measurements of heat transfer coefficient during flow boiling of two HFOs inside a circular minichannel
No full textNanofluids application in direct absorption solar collectors: review and numerical model
No full textThe application of nanofluids has the potential to solve technical key issues in many solar thermal engineering systems. Recent literature indicates that nanofluids offer unique advantages over conventional fluids. Nanofluids are made of solid nanoparticles suspended in a liquid. These particles enhance optical properties of the liquid suspension, increasing the efficiency in the conversion of solar radiation into thermal energy. This study investigates the application and challenges of nanofluids in solar energy systems. The main literature on numerical models of nanofluids in solar thermal energy is here presented. In particular, the attention has been focused on nanofluid-based direct absorption solar collectors (DASC). Based on this review, a new model of a nanofluid-based direct absorption solar receiver for a concentrating solar collector has been proposed and then applied to predict the performance of a receiver with single-wall carbon nanohorns aqueous suspension
Condensation heat transfer and pressure drop in a single minichannel with R1234ze(E) and other refrigerants
No full textCondensation heat transfer and two-phase frictional pressure drop in a single minichannel with R1234ze(E) and other refrigerants
No full textR1234ze(E), trans-1, 3, 3, 3-tetrafluoropropene, is a fluorinated propene isomer which may
be a substitute of R134a for refrigeration applications. R1234ze(E) has a much lower GWP
100-years
than that of R134a. In this paper, the local heat transfer coefficient during condensation of R1234ze(E) is investigated in a single minichannel, horizontally arranged, with
hydraulic diameter equal to 0.96 mm. Since the saturation temperature drop directly af-
fects the heat transfer rate, the pressure drop during adiabatic two phase flow of R1234ze(E)
is also measured. Predictive models are assessed both for condensation heat transfer and
pressure drop. A comparative analysis is carried out among several fluids (R1234ze(E), R32,
R134a and R1234yf) starting from experimental data collected at the same conditions and
using the Performance Evaluation Criteria (PEC) named Penalty Factor (PF) and Total
Temperature Penalization (TTP) to rank the tested refrigerants in forced convective
condensatio
EXPERIMENTAL STUDY OF CONDENSATION INSIDE A SQUARE MINICHANNEL: EFFECT OF CHANNEL ORIENTATION
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