INTERNATIONAL JOURNAL OF REFRIGERATION - Laboratory for refrigeration and district energy

Evaluating R450A as a drop-in replacement for R134a in household heat pump tumble dryers

lahde — Wed, 22 Dec 2021 07:06:00 +0000

An investigation into the applicability of R450A as a replacement for R134a in heat pump tumble dryers was made in collaboration with Gorenje, d.o.o.

This paper presents the results of an experimental study evaluating R450A as a possible drop-in replacement for R134a in household heat pump tumble dryers. A heat pump installed in a tumble dryer was equipped with pressure and temperature sensors, and electrical power meters. First, the energy consumption according to the IEC 61121 standard was evaluated using R134a. Then the refrigerant was replaced with R450A and a new optimal charge was determined. The tumble dryer performance was then evaluated again and compared with the previous results. It was found that R450A is a suitable drop-in replacement for R134a in heat pump tumble dryers. The total energy consumption of the tumble dryer decreased by 3.7% and several other benefits such as a better utilized evaporator, and a lower refrigerant temperature at the compressor outlet were observed. Additionally, the refrigerant charge CO₂ equivalent was reduced by more than 50%. However, the duration of the drying cycle increased by 4.2%, due to a slightly lower condensing temperature in the heat pump.

Link: https://www.sciencedirect.com/science/article/pii/S0140700721001195

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Impact of neglecting the variations in the relative surface roughnesses of capillary tubes on the accuracy of a capillary tube model

lahde — Tue, 04 May 2021 08:18:17 +0000

Existing capillary tube models rely heavily on friction factor correlations, which enable the specification of the relative surface roughness. However, measuring the surface roughnesses of capillary tubes is a time consuming task, which requires specialized equipment. In this research we have developed a friction factor correlation, which does not require that the surface roughness is measured and has a comparable accuracy to existing correlations.

ABSTRACT:

Capillary tube models frequently rely on friction factor correlations, which are functions of the Reynolds number, and the tube’s relative surface roughness. However, this study shows that explicitly correlating the friction factor with the relative surface roughness is unnecessary, at least when the latter is lower than 0.0025. To test this a correlation was fitted directly onto friction factor measurements obtained in an adiabatic flow of R600a, through nine capillary tubes from three manufacturers. The friction factor was correlated exclusively as a function of the Reynolds number, meaning that the impacts of the relative surface roughnesses on the friction factor were averaged, and included only implicitly. This correlation, and an existing correlation, which allows the specification of the relative surface roughness, were then implemented in a capillary tube model. The accuracy of the capillary tube model, using either the developed or existing correlation was then evaluated by comparing the results to experimental data. No significant decrease in the accuracy of the model was observed when the relative surface roughnesses, ranging from 0.0012 to 0.0025, were not individually specified for each capillary tube. To further verify this, the procedure was repeated on a larger database of friction factor and mass flow rate measurements, which also consisted of data from independent studies. Similar conclusions were drawn, as the average accuracy of the model differed only by 1 to 3% among different studies, and not always in favor of the correlation where the relative surface roughnesses could be specified.

LINK: https://www.sciencedirect.com/science/article/pii/S014070072100147X#ack0001

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Two-phase flow patterns in adiabatic refrigerant flow through capillary tubes

lahde — Wed, 08 Apr 2020 06:54:04 +0000

Recently published paper in International Journal of Refrigeration reports on flow visualization of the refrigerant R600a through a capillary tube (link). (Open access link (50 days))

Abstract: This paper presents the results of an experimental study of the flow visualization of the refrigerant R600a through a capillary tube with an internal diameter of 0.8 mm made from an FEP (fluorinated ethylene propylene) polymer that was installed in a small-scale vapor-compression refrigeration system. The main purpose of the study was to determine the flow patterns of the refrigerant flow in capillary tubes under different operating conditions and to verify whether the results obtained using an FEP tube are representative for the flow of a refrigerant in copper capillary tubes. The results of the study revealed that the two-phase refrigerant flow in the FEP capillary tubes consists predominantly of a heterogenous flow composed of slugs, which continuously expand downstream. It was also found that at higher inlet pressures the frequency at which the vapor inception occurs is higher than at lower pressures. In addition, an experimental study of the effect of local or global imperfections on the inner surface of the FEP tube was performed. In some cases, the imperfections increased the number of vapor bubble inception points, which caused a homogenization of the two-phase flow.

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Very Commended Paper Award, International Journal of Refrigeration

lahde — Thu, 05 Sep 2019 17:11:43 +0000

The paper

Electrocaloric refrigeration: Thermodynamics, state of the art and future perspectives (M. Ozbolt, A. Kitanovski, J. Tusek, A Poredos)

was awarded by International Journal of Refrigeration for a Very Commended Paper.

The paper is available here.

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Numerical modelling and experimental validation of a regenerative electrocaloric cooler

lahde — Fri, 22 Feb 2019 15:52:24 +0000

The article was publishe in the International Journal of Refrigeration (link)

Abstrakt

This paper reports on the research and development of a cooling device with an active electrocaloric regenerator (AER) based on the bulk ceramic material (1-x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ (PMN-100xPT). For the purposes of the study a 2D transient numerical model of the AER was developed. This model makes it possible to investigate the cooling characteristics of a device with an AER while considering the effect of the hysteresis of the electrocaloric material and the effect of the electric-energy recovery related to the polarization/depolarization process. The results of the numerical analyses show that the degree of electric energy recovery has a major impact on the efficiency of the device. By considering an idealised system for electric-energy recovery the energy efficiency (expressed by the coefficient of performance) of the device could be increased by up to ten times. A validation of the numerical model was performed through the design, construction and experiments on an improved AER cooling device. The results revealed a maximum specific cooling power of 16 W kg⁻¹ and a maximum temperature span of 3.1 K for the new device.

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