Two-phase Refrigerant Research Articles

Green Cooling of High Performance Microprocessors: Parametric Study Between Flow Boiling and Water Cooling

Due to the increase in energy prices and spiralling consumption, there is a need to greatly reduce the cost of electricity within data centers, where it makes up to 50% of the total cost of the IT infrastructure. A technological solution to this is using on-chip cooling with a single-phase or evaporating liquid to replace energy intensive air-cooling. The energy carried away by the liquid or vapor can also potentially be used in district heating, as an example. Thus, the important issue here is “what is the most energy efficient heat removal process?” As an answer, this paper presents a direct comparison of single-phase water, a 50% water–ethylene glycol mixture and several two-phase refrigerants, including the new fourth generation refrigerants HFO1234yf and HFO1234ze. Two-phase cooling using HFC134a had an average junction temperature from 9 to 15 °C lower than for single-phase cooling, while the required pumping power for the central processing unit cooling element for single-phase cooling was on the order of 20–130 times higher to achieve the same junction temperature uniformity. Hot-spot simulations also showed that two-phase refrigerant cooling was able to adjust to local hot-spots because of flow boiling’s dependency on the local heat flux, with junction temperatures being 20 to 30 °C lower when compared to water and the 50% water–ethylene glycol mixture, respectively. An exergy analysis was developed considering a cooling cycle composed by a pump, a condenser, and a multimicrochannel cooler. The focus was to show the exergetic efficiency of each component and of the entire cycle when the subject energy recovery is considered. Water and HFC134a were the working fluids evaluated in such analysis. The overall exergetic efficiency was higher when using HFC134a (about 2%), and the exergy destroyed, i.e., irreversibilities, showed that the cooling cycle proposed still have a huge potential to increase the thermodynamic performance.

Consistent initialization of system of differential-algebraic equations for dynamic simulation of centrifugal chillers

For simulation of centrifugal chiller system with differential algebraic equations, consistent initial conditions are difficult to obtain due to the two-phase refrigerant cycle and the connections between several components. Existing rigorous initialization methods are not suitable for the particular problems in centrifugal chiller simulation, such as the surge instability. In this paper, the dynamic model of a water-cooled centrifugal chiller is developed in Modelica with Dymola and the TLK/IfT Library (TIL). A preprocessing scheme and a direct initialization method are proposed for the consistent initialization of chiller DAE system. Simulation results have shown the effectiveness of the proposed modelling framework and the initialization method.

Two-phase refrigerant flow instability analysis and active control in transient electronics cooling systems

Two-loop refrigeration systems are being explored for two-phase cooling of ultra high power electronic components. For effective and efficient thermal management of electronic systems, active control methods are desired to suppress inherent flow instabilities especially in transient applications. This paper presents a framework for the transient analysis and active control of pressure-drop flow instabilities under varying imposed heat loads. The external effects on boiling flow characteristics and the boiling oscillatory flow responses to transient heat load changes are studied. Flow instability margins can be quantitatively predicted from an analytical two-phase flow model. In addition, the effects of wall thermal inertia on flow oscillations is systematically investigated. Based on the theoretical analysis of oscillatory flow boiling of refrigerants, a set of active control schemes are developed and studied to suppress flow oscillations and to increase the critical heat flux. With the available control devices – inlet valve and supply pump – different active control schemes are studied to improve the transient two-phase cooling performance.

Characterisation of ice and THF hydrate slurry crystal size distribution by microscopic observation method

The objective of the present work is to characterise, using an experimental microscopic observation device, the crystal size distribution (CSD) of ice and tetrahydrofuran (THF) hydrate slurries, which can be used as two-phase refrigerant fluids for cold transportation and storage in the field of secondary refrigeration. Three different solutes (ethanol, propylene glycol and sodium chloride) were used to form ice slurries. The studied hydrate was THF hydrate since it is stable at atmospheric pressure and in a temperature range suitable for refrigeration and air-conditioning applications. Experimental ice and THF hydrate slurry CSDs were compared to classical normal (Gaussian), log-normal, gamma, and Weibull (Rosin–Rammler) distributions. A comparison between ice and THF hydrate slurry CSD results showed that hydrate slurries were composed of crystals approximately 1.5 times smaller than ice crystals. The present results were shown to be useful input parameters for modelling thermophysical properties of slurry systems.

Thermal Characterization of Interlayer Microfluidic Cooling of Three-Dimensional Integrated Circuits With Nonuniform Heat Flux

It is now widely recognized that the three-dimensional (3D) system integration is a key enabling technology to achieve the performance needs of future microprocessor integrated circuits (ICs). To provide modular thermal management in 3D-stacked ICs, the interlayer microfluidic cooling scheme is adopted and analyzed in this study focusing on a single cooling layer performance. The effects of cooling mode (single-phase versus phase-change) and stack/layer geometry on thermal management performance are quantitatively analyzed, and implications on the through-silicon-via scaling and electrical interconnect congestion are discussed. Also, the thermal and hydraulic performance of several two-phase refrigerants is discussed in comparison with single-phase cooling. The results show that the large internal pressure and the pumping pressure drop are significant limiting factors, along with significant mass flow rate maldistribution due to the presence of hot-spots. Nevertheless, two-phase cooling using R123 and R245ca refrigerants yields superior performance to single-phase cooling for the hot-spot fluxes approaching ∼300 W/cm2. In general, a hybrid cooling scheme with a dedicated approach to the hot-spot thermal management should greatly improve the two-phase cooling system performance and reliability by enabling a cooling-load-matched thermal design and by suppressing the mass flow rate maldistribution within the cooling layer.

On reducing evaporator superheat nonlinearity with control architecture

Evaporator superheat control is an important aspect of the operation of refrigeration and air conditioning systems; since the majority of cooling in these systems occurs through evaporation of two-phase refrigerant, the energy efficiency is dramatically improved by reducing the amount of superheat present. However, allowing refrigerant to leave the evaporator without completely vaporizing risks catastrophic damage to the compressor, so superior control is required at low superheat levels. One of the most significant challenges present in this control problem is the presence of significant nonlinearities in the response from the control input, e.g. expansion valve position, to evaporator superheat. This paper reveals how a particular control architecture inherently compensates for both the static and dynamic nonlinearities that dominate the valve-to superheat transient response. Furthermore, the control implementation only requires temperature measurements, which are frequently available in ordinary HVAC systems. Experimental results confirm the reduction of nonlinearities using the proposed approach, and the authors discuss the effect of actuator limitations on the nonlinearity compensation.

증발기 입구 배관의 구조 개선을 통한 냉장고 냉매 소음 저감

This research is focused on the experimental study of the noise induced by two-phase refrigerant flow in the evaporator. The two-phase flow in the evaporator has various flow patterns. The effects of two-phase flow pattern's characteristics on the noise of the evaporator are investigated experimentally. The experimental data shows that the generated noise is mainly related to the layout of the pipe and the certain two-phase flow patterns such as the churn and slug flow. Based on these results, we removed the unnecessary vertical pipe and changed the pipe diameter of the evaporator - inlet into small one in order to avoid the intermittent flow condition. The noise level of newly-designed inlet-pipe of the evaporator was measured experimentally by refrigerant-supplying equipment and compared with that of conventional one.

Improved methodologies for simulating unitary air conditioners at off-design conditions

Models for unitary air conditioners and heat pumps typically have difficulties in providing accurate predictions for the following situations: 1) charge modeling over a large range of charge levels, 2) two-phase refrigerant entering fixed-orifice expansion devices, 3) two-phase refrigerant entering compressors, and 4) non-uniform refrigerant mass flow allocation among evaporator circuits at low indoor air flow rates. This paper gives descriptions and assessments of improved modeling methodologies for all four issues. The study utilizes data obtained from extensive testing at a wide range of operating conditions for two R-410A units and one R-407C unit.

Experimental validation of void fraction models for R410A air conditioners

This paper presents the combined void fraction models on the basis of flow pattern to choose suitable models for calculating the two-phase refrigerant mass of R410A air conditioners, and the model is validated experimentally. The existing single void fraction models are also evaluated experimentally. The Taitel–Dukler flow pattern map is recommended to use in the combined void fraction models. The results presented herein show that the calculated refrigerant mass using the combined void fraction models agrees relatively better with the measured refrigerant mass than that of the single void fraction models. T–B model (for intermittent flow) and Premoli model (for annular flow) are recommended for calculating the two-phase refrigerant mass in evaporator, and T–B model (for intermittent flow) and Harms model (for annular flow) are also indicated for calculating the two-phase refrigerant mass in condenser.

Improving two-phase refrigerant distribution in the manifold of the refrigeration system

This study presents an experimental investigation to improve the two-phase refrigerant distribution in the manifold of the refrigeration (R-134a) system. The tests were conducted at the nominal inlet pressure of 173.5 kPa and mass flux of 130–145 kg/m 2 s. Three different distributor configurations (Venturi tube connected with a smooth tube, Venturi tube connected with an internally–spirally micro-finned tube, and distributor within a roller) were selected. In the experiment, the effects of the outer-wall temperatures and pressures of the exits in the manifold, the cooling capacity ( C) and coefficient of performance (COP) on two-phase flow distribution for the present study were displayed and discussed. In order to understand that flow distribution of two-phase refrigerant in the manifold, the flow visualization at the exit manifold of the distributor was made. It was found that the Venturi tube connected with the internally–spirally micro-finned tube got the best distribution efficiency, and resulted in the best coefficient of performance.

Reproductive Accuracy of Generic Omega Parameter Safety Valve Two Phase Critical Mass Flow Models Based on Water and Refrigerant R‐134a Data

AbstractThe modified Omega method included in ISO/FDIS 4126‐10 provides the highest accuracy compared to other modified models frequently used in practice and academia. At the same time, it allows for an exceptionless oversizing of the necessary relief area under all test conditions. Nevertheless, the average or statistical reproductive accuracy is characterized by an unacceptably high logarithmic scatter of about 80 % for fluid dynamic critical steam/water and two‐phase refrigerant R‐134a flow. When applying the method to sizing situations with predicted mass flow qualities in excess of 1 % an acceptably modest oversizing is maintained.

Adiabatic Two-Phase Pressure Drop of Refrigerants in Small Channels

The adiabatic pressure drop of two-phase refrigerant flow in small channels has been investigated. A rectangular channel with d h = 148.0 μm has been tested with four refrigerants: R134a, R410A, propane (R290), and ammonia (R717). These data have been combined with data taken from five different channels, with d h varying from 70 μm to 305 μm, of R134a. The measured pressure drops have been compared to many published separated-flow and homogeneous pressure drop models. A new correlation for C, the Chisholm parameter, has been developed based on the Reynolds number of the vapor phase (which contains the majority of the kinetic energy) and the dimensionless grouping ψ—a ratio of viscous to surface tension effects taken from the analysis of capillary flow performed by Sou and Griffith (1964). This allows the new correlation to account for the varying fluid properties (including surface tension) that are found in the different refrigerants. The new correlation takes flow regime into account by means of a Weber number based flow transition criteria, following the flow map of Akbar et al. (2003).

Refrigerant Distribution in Minichannel Evaporator Manifolds

The effects of geometry and operating conditions on the distribution of refrigerant R-410A in heat exchangers with horizontal manifolds and vertically oriented minichannels were experimentally investigated to provide the essential design information for the minichannel evaporators. Flow visualization in the horizontal manifold showed that the flow patterns of the two-phase refrigerant before and after the liquid-vapor transition are in stratified flow for the end-inlet location and in bubbly and stratified flows for the side-inlet location. Test results showed that the normalized standard deviation of the liquid mass flow rate, which indicates the degree of maldistribution, changes from 0.088 to 0.263 with manifold inlet location changes, from 0.034 to 0.141 with manifold inlet mass flow rate changes, from 0.037 to 0.082 with tube number changes, and from 0.027 to 0.055 with tube pitch changes. The normalized standard deviation of the liquid mass flow rates showed that the liquid refrigerant flow distribution is strongly affected by the manifold inlet location and the manifold inlet mass flow rate but is primarily independent of tube pitch. The side-inlet location showed a better liquid refrigerant flow distribution than the end-inlet location by more effectively mixing of the liquid and the vapor refrigerant from the inlet. Therefore, the side-inlet location is preferred to the end-inlet location.

Modelling of the available latent heat of a CO 2 hydrate slurry in an experimental loop applied to secondary refrigeration

The present study investigates the suitability of CO 2 hydrate for a use as phase change material in two-phase secondary refrigeration. Unlike the generation of the classical two-phase refrigerants, power limited by mechanical parts, hydrate slurry production has the advantage of being performed using a nonmechanical process. Nevertheless, in order to be efficient, the hydrate slurry needs to fulfil two major conditions: a high latent heat of melting of the solid phase and appropriate flowing conditions of the slurry carrying a sufficient amount of solid. Consequently, in the present work, multi-cycle differential scanning calorimetry (DSC) measurements were performed and confirmed a value of CO 2 hydrate dissociation enthalpy of approximately 500 kJ k g w − 1 , one and a half higher than that of ice ( 333 kJ k g w − 1 ) . Moreover, an experimental loop made it possible to study the CO 2 hydrates in suspension in a carrying liquid and to model the available enthalpy of the system related to the solid fraction of the slurry.

Reverse heat transfer and re-condensation phenomena in non-adiabatic capillary tubes

A numerical simulation model for lateral capillary tube-suction line heat exchangers is presented here to analyze its performance characteristics in small vapour compression refrigeration systems (e.g. domestic refrigerators). Appropriate heat transfer correlations have been used to illustrate the reverse heat transfer and re-condensation phenomena inside the tubes. Some convergence problems were encountered during the execution of the model when lower vapour temperature inside the suction line caused the two-phase refrigerant inside the capillary tube to re-condense within the heat exchange region. Therefore, a relationship between the re-condensation phenomenon and the divergence problem has been analyzed in the paper. The modelling was performed with two refrigerants, namely HFC-134a and HC-600a. Further, a simple theoretical equation has been developed to express the re-condensation phenomenon in non-adiabatic capillary tubes.

Rheological study of TBAB hydrate slurries as secondary two-phase refrigerants

In the refrigeration field, the use of ice slurries (suspensions of ice) as secondary refrigerant fluids is spreading. This technology could be extended to air-conditioning systems provided that one can propose a phase-change material (PCM) with melting temperature belonging to the range [+6 °C; +12 °C]. Aqueous solutions of tetra butylammonium bromide, which crystallise under atmospheric pressure and temperatures between 0 and +12 °C, into ice-like compounds (hydrates), seem to be good candidates for air-conditioning applications. We focus here on the rheological properties of TBAB hydrates suspensions and propose an experimental determination of their flow behaviour through flow rate and pressure drop measurements. In laminar regime, we observe Bingham behaviour. Apparent viscosity and yield shear stress are determined for different hydrate contents.

Thermoeconomic optimisation of the condenser in a vapour compression heat pump

In the paper, the design optimisation of a heat exchanger is discussed, using a thermoeconomic approach. The investigation is referred to the tube-in-tube condenser of a conventional vapour-compression heat pump, with a two-phase refrigerant flowing in the inner tube and a single-phase fluid flowing in the annulus. A cost function to be minimised is introduced, defined as the sum of two contributions: (i) the amortisation cost of the condenser, related to the heat exchange area; (ii) the operating cost of the electric-driven heat pump in which the heat exchanger will work, depending on the overall exergy destruction rate in the system. In the paper, this latter contribution is related to the local irreversibility rate in the condenser, using the so-called structural approach. The optimal trade-off between amortisation and operating cost is therefore investigated, by minimising the above-mentioned cost function. A numerical example is discussed, in which, for a commercial heat exchanger, the design improvements needed to obtain a cost-optimal configuration are investigated. The analysis is carried out for three different refrigerants: R22, R134a and R410A.

Two-phase flow distribution in compact heat exchanger manifolds

In this paper an experimental study to investigate two-phase flow distribution in compact heat exchanger manifolds is presented. An experimental test rig to measure the flow distribution in realistic manifold geometries and under relevant operating conditions was built. To elucidate the effect on the flow distribution, the following factors are investigated: vapour fraction at the manifold inlet, heating load on the heat exchanger tubes, diameter of the manifold and manifold inlet tube length. The experimental system simulated an evaporator for an automobile air conditioner with a design cooling capacity of 5 kW. From the horizontal manifold, the two-phase refrigerant flow was distributed in 10 parallel heat exchanger tubes. The orientation of the test rig could be varied to measure both upward and downward flow out of the manifold. To ensure similarity to an actual heat exchanger, counterflowing water was used as heat source in the evaporator section. The experimental setup made it possible to measure the heat added to the evaporating refrigerant in each tube by using separate water flow circuits. Refrigerant R134a was used as refrigerant.

Two-Phase Refrigerant Flow Distribution in a Multipass Evaporator with Vertical Upward Main Tube

Two-Phase Refrigerant Flow Distribution in a Multipass Evaporator with Vertical Upward Main Tube

Determining the optimal configuration of a heat exchanger (with a two-phase refrigerant) using exergoeconomics

In this paper, the exergoeconomic theory is applied to a heat exchanger for optimisation purposes. The investigation was referred to a tube-in-tube condenser with the single-phase fluid to be heated flowing in the inner annulus and the two-phase refrigerant flowing in the external annulus. First, the irreversibility due to heat transfer across the stream-to-stream temperature-difference and to frictional pressure-drops is calculated as a function of two design variables: the inner-tube's diameter and the saturation temperature of the refrigerant, on which the heat-exchange area directly depends. Then, a cost function is introduced, defined as the sum of two contributions: the amortisation cost of the condenser under study and the operating cost of the conventional electric-driven heat-pump in which this component will have to work. The latter contribution is directly related to the overall exergy destruction rate in the plant, whereas the amortisation cost mainly depends on the heat-exchange area. So, design optimisation of the device can be performed by minimising this cost function with respect to the selected design variables. The so-called structural approach (Coefficient of Structural Bond) is used in the optimisation, in order to relate the local irreversibility in the condenser to the overall exergy destruction rate in the heat-pump plant. A numerical example is discussed, in which, for a commercial heat-exchanger, the design improvements needed to obtain a cost-optimal configuration are investigated. The results show that significant improvements can be obtained with respect to devices based on conventional values of the design parameters.