1,721,345 research outputs found
Flow boiling of halogenated refrigerants at high saturation temperature in a horizontal smooth tube
No full textSeveral correlations are available in the open literature for computing the heat transfer coefficient during
flow boiling inside plain channels. With respect to halogenated refrigerants, these correlations are usually
compared to data taken in a limited range of evaporation temperature and reduced pressure. More
recently, the adoption of new refrigerants, such as high pressure HFCs and carbon dioxide, requires to largely
extend the pressure range of application of such correlations. Besides, the design of evaporators for
some heat pumping applications, where temperatures are set at higher values as compared to usual evaporating
temperatures in air-conditioning equipment, requires proper validation of the computing methods.
The present paper aims at comparing four well-known predicting models to a new database collected
during flow boiling of HCFC (R22) and HFC refrigerants (R134a, R125 and R410A) in a horizontal 8 mm
internal diameter tube. This database is characterized by saturation temperature ranging between
25 C and 45 C, reduced pressure spanning between 0.19 and 0.53. Mass velocity ranges between 200
and 600 kg m2 s1 and heat flux between 9 and 53 kW m2.
Evaporating heat transfer coefficients of halogenated refrigerants at such high temperatures have not
been reported in the open literature so far. The discussion of the results will enlighten some similarities
with experimental trends presented in the literature for evaporating carbon dioxide.
Two models tested here show good prediction capabilities of the present experimental data, but not for
all the data sets in the same way. For the purpose of practical use, a simple modification of the correlation
by Gungor and Winterton [1] is proposed, showing that this is able to catch the experimental trends of
the present database with good agreement
Investigation of dryout during flow boiling in a single microchannel under non-uniform axial heat flux
No full textThis paper presents an experimental investigation on the dryout during flow boiling of R245fa, R134a and R32 inside a 0.96 mm diameter single circular microchannel. In the present tests, the channel is not electrically heated; instead, the flow boiling is achieved by means of a secondary fluid (water) resulting in a non-uniform distribution of the heat flux along the channel. Since the wall temperature is limited by the temperature of the secondary fluid, the onset of dryout is detected by means of the standard deviation of the temperature readings in the wall. The wall temperature in fact displays larger fluctuations in the zone where dryout occurs, which are related to the presence of the liquid film drying up at the wall with an oscillating process. These temperature fluctuations are detected by means of the standard deviation of the wall temperature. These fluctuations never appear during flow boiling at low vapor qualities; they also disappear in the post-dryout zone. Experimental values of dryout quality measured with the above method are reported in this paper for mass velocity ranging between 100 and 900 kg m-2 s -1, during annular flow. The present data, which covers a wide range of reduced pressure (between 0.05 and 0.34), has been compared against some critical heat flux models available in the literature. Since in the present data the heat flux is not uniform along the channel, each dryout point is characterized by its own boiling story. The actual heat flux profile can be used when comparing with the model by Revellin and Thome [1]. For other models, which are developed for uniformly heated microchannels, the predicted CHF is compared to the average experimental heat flux in the channel
An improved procedure for the experimental characterization of optical efficiency in evacuated tube solar collectors
No full textThe standard EN 12975-2 provides guidelines for testing solar collectors both in stationary and quasi-dynamic conditions. The second test method allows the optical efficiency of flat-plate collectors and even evacuated tube collectors to be determined by applying the extended multiple linear regression. However, in the case of tubular shape collectors, the available procedure requires a large number of data, above all for the determination of the transversal incidence angle modifier, which is the parameter describing the optical response of the absorber tube to the direct beam on the plane normal to the tube axis. Here, an improved procedure to determine the transversal incidence angle modifier is presented and validated against experimental data. For this purpose, efficiency tests in quasi-dynamic conditions have been performed following the standard EN 12975-2 on a U-tube evacuated tubular collector, using a cylindrical absorber, both with and without external CPC (compound parabolic concentrator) reflectors. The validation has been performed by comparing the efficiency curve and the curve of incidence angle modifier to the ones that are obtained by means of other available methods. The main advantages of the present new procedure are the followings: it provides a continuous curve of the incidence angle modifier and it does not require to subdivide the incidence angle range in many intervals. Therefore, it does not require a minimum number of data points for each data subset and thus it is less demanding in terms of required number of tests. © 2011 Elsevier Ltd
Development and experimental validation of a numerical model for flat-plate solar collectors
No full textFlow boiling of a new low-GWP refrigerant inside a single square cross section microchannel
Full text linkIn this work new experimental heat transfer coefficients measured during flow boiling of HFO-1234ze(E) in a horizontally-assembled square cross section microchannel having an hydraulic diameter of 1.23 mm are presented. The test runs have been performed at around 30°C saturation temperature, correspondent to 5.8 bar, with mass flux ranging between 300 kg m-2 s-1 and 500 kg m-2 s-1. As a peculiar characteristic of the present technique, the heat transfer coefficient is not measured by imposing the heat flux; instead, the boiling process is governed by controlling the inlet temperature of the heating secondary fluid. On this regard the present data is new and original since the large majority of data in the literature is taken by means of Joule effect heating. The heat transfer coefficients are compared against two predicting models available in the literature. Finally, the local heat transfer coefficients measured during flow boiling of R1234ze(E) inside the square cross section microchannel are compared against the values measured with the same refrigerant in a 0.96 mm diameter circular microchannel, with same surface roughness, with the aim of investigating the effect of channel shape on the heat transfer process
Condensation in minichannels: experimental investigation and numerical modelling
No full textThis paper presents an overview of the most recent research works on condensation heat transfer inside minichannels with halogenated refrigerants. The first part is focused on the experimental results available in the scientific literature and the effect of the main parameters (channel diameter, mass velocity, vapour quality, saturation temperature, heat flux) on convective condensation is discussed. In particular the effect of channel shape is treated, showing that at low mass velocity some peculiar characteristics of microscale condensation can be observed. In the second part, numerical models of condensation in minichannels are presented. These models should be able to simulate the heat transfer process resolving the momentum and energy equations and without any empirical input. One important feature of numerical modelling is to provide an insight on the heat transfer mechanisms that may be difficult to get from the experimental investigation. On
the other hand, it should be noticed that such numerical models, due to the heavy computational effort required, are still far to be widely applied in heat exchanger design
Hygro-thermal performance of ventilated underground cavities
No full textThe risk assessment regarding overheating in underground structures is becoming quite common among the designers, when internal heat gains are severe or they occur for a long time. Typical is the case of subway tunnel networks, but many other recent examples may be easily found.
Quite often however, not only the air temperature pattern is relevant, but also humidity may play an important role, especially if underground water infiltrations are not negligible. In fact, water evaporation or condensation may significantly affect the temperature profiles; on the other hand, high humidity ratios may hamper some activities performed in the cavities.
The present paper aims to present a design tool to simulate the hygro-thermal behaviour of ventilated underground cavities. The model can be used to simulate steady-state or transient conditions: it solves the heat and mass transfer equations in the cavity, providing also a criterion for the evaluation of the fog in the air
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