Capillary Tube-suction Line Heat Exchanger Research Articles

Modeling of contact thermal resistance in lateral capillary tube-suction line heat exchangers with experimental validation and investigation

This paper studied the lateral capillary tube-suction line heat exchanger (CTSLHX) with different contact types and different geometries. An improved one-dimensional model of the CTSLHX was established, using the CFD model to derive the contact thermal resistance between the capillary tube and the suction tube. An experimental bench was built to test the steady-state heat transfer and flow performance. It was found that the model predicted 87.6 % and 98.9 % of the measured mass flow rate within the error band of ± 10 % and ± 15 %. The prediction error of the CTSLHX heat transfer effectiveness was within ± 10 %. Parametric effects of different operating conditions and structural parameters on the performance of CTSLHX were investigated. Furthermore, experiments were carried out to study the temperature distribution along the transition section between the exit of the capillary tube and the evaporator. The unique temperature increase reveals that it is beneficial for reducing noise by wrapping a layer of rubber around the second increaser fitting.

Dynamic modelling of the steady state and load processing operation of a domestic refrigerator cooled through natural convection

A dynamic model of a domestic refrigerator is developed in this study and is used to analyse both the steady state (cyclic) and warm load cooling (load processing) operation. The compressor is modelled semi-empirically using loss parameters, while the capillary tube is modelled by incorporating both friction and momentum pressure drop. Heat exchangers (condenser and evaporator) are modelled as tubes divided into finite volume cells. The capillary tube-suction line heat exchanger is treated by connecting part of the capillary tube to the suction line via a heat resistor. The cabinet model consists of submodels for walls, air and shelves connected in a network to allow for thermal interaction. All the components are then arranged in a cycle, and cyclic operation is validated at 32 °C using a fixed speed compressor. A sensitivity analysis is then carried out by varying the compressor speed. In the second part of the study, the model is modified to include the effect of placing a warm load of water in the refrigerator and simulating the subsequent processing operation. This is validated using an experiment in which 20 L of water at 27 °C is placed inside the Fresh Food compartment. Results show that the power consumption value is captured well, with the average power consumption deviating less than 10% for all tests. The energy consumption is within ±5% for all tests. For load processing, the deviation in water temperature was a maximum of 1.2 °C for the whole cooling period.

Study on Flow Behavior and Heat Exchange Characteristics of a Capillary Tube-Suction Line Heat Exchanger

ABSTRACTThe present research is to develop a homogenous mathematical model to simulate capillary tube-suction line heat exchanger (CT-SL HX) based on the fundamentals of conservations of mass, momentum and energy with comprehensive experimental result validations. The computer model is fully validated by 72 experimental data with error bands of ±15%, ±2°C and ±35% on the mass flow rate prediction, the suction pipe outlet temperature, and the heat exchange estimation respectively. The results suggest that the internal diameter of the capillary tube, and the heat-transfer length of the CT-SL HX have demonstrated an apparent impact on the capillary tube outlet conditions and heat transfer across the segment.

Numerical simulation of two phase refrigerant flow through non-adiabatic capillary tubes using drift flux model

This research presents a drift flux model to simulate steady state refrigerant flow through horizontal lateral straight capillary tube-suction line heat exchangers with refrigerant R134a as a working fluid. The geometry of capillary tube is divided into two regions: An entrance length and lateral heat exchanger length. Metastable flow phenomenon is neglected in this work. To better study of fluid behavior in non-adiabatic capillary tube, according to the refrigerant state at the entrance of heat exchanger region, three modes were studied and the fundamental equations were solved for all three modes, then the results were analyzed. The present data using a drift flux model were validated with previous numerical work with a homogeneous model with obtaining reasonable agreement. By considering the slip ratio effects in drift flux model, therefore, this model could able to predict refrigerant flow behavior more accurately than other two-phase flow models in non-adiabatic flow condition through capillary tube.

Development of a continuous empirical correlation for refrigerant mass flow rate through non-adiabatic capillary tubes

Capillary tube, suction line heat exchangers (CT-SLHX) are widely used as expansion devices in small-capacity refrigeration and air-conditioning systems to enhance the refrigeration capacity and ensure that superheated refrigerant vapor enters the compressor. To calculate the mass flow rate through a capillary tube, a reliable non-adiabatic capillary tube model is necessary. Most previous correlations were developed separately for subcooled liquid inlet conditions and for saturated two-phase inlet conditions; so the models are not continuous at the saturated liquid point. An empirical model that is continuous at the saturated liquid point was developed and is introduced herein with a new dimensionless π parameter. This new empirical model is validated using experimental measurements available in the literature for the refrigerants R-134a, R-600a, R-410A, R-152a, and R-22. The new correlation shows good agreement with the experimental data.

A modified approach for numerical simulation of capillary tube-suction line heat exchangers

This paper presents a modified approach for numerical simulations of capillary tube-suction line heat exchangers (CTSLHE). This approach is based on an approximate solution methodology previously presented. This methodology models the expansion process separately from the heat transfer process, giving rise to two sub-models, namely hydrodynamic and thermal sub-models. In the present paper, a two-step predictor–corrector model is proposed, one for the refrigerant mass flux and another for the refrigerant enthalpy path. Simulation results of mass flow rate and suction line outlet temperature were compared with experimental data, empirical correlations and well ranked distributed models. The simulation results showed good agreement with experimental data, with almost all mass flow rate results were within ±20% error bands and most suction line outlet temperature results within ±5°C. Reasonable agreement was observed when the predicted results of mass flow rate, critical pressure and suction line outlet temperature were compared against the results from well ranked literature numerical models. Both, the influence of the capillary tube inner diameter and the suction line inner diameter over performance parameters were also investigated. No convergence problems were observed during the simulations.

Numerical simulation of the effects of a suction line heat exchanger on vapor compression refrigeration cycle performance

Most modern refrigerators incorporate heat transfer between the refrigerant in a capillary tube and the refrigerant in a suction line. This heat transfer is achieved by a non-adiabatic capillary tube called a capillary tube-suction line heat exchanger and is supposed to improve the performance of the small vapor compression refrigeration cycle by removing some enthalpy of the refrigerant at the evaporator entrance. To investigate the effects of this heat transfer on the refrigeration cycle, a computer program was developed based on conservation equations of mass, momentum, and energy. The non-adiabatic capillary tube model is based on a homogeneous two-phase flow model. The simulation results show that both the location and length of the heat exchange section influence the coefficient of performance (COP) as well as the cooling capacity. It is noteworthy that the influence was not monotonic; that is, the performance may be deteriorated under certain conditions.

Development of empirical correlations for non-adiabatic capillary tube based on mechanistic model

Capillary tubes play a vital role in the refrigeration cycle, as they reduce the temperature of the refrigerant. The non-adiabatic capillary tube (capillary tube-suction line heat exchanger: CT-SLHX) is widely incorporated into small refrigeration systems to enhance the cooling performance and ensure that only the refrigerant vapor enters the compressor. It is necessary to develop an accurate and fast-running non-adiabatic capillary tube model, as the performance of the refrigeration cycle simulation tool is largely dependent on the accuracy and run-time its component models. In the present study, a reliable mechanistic model was used to generate reference data. An empirical model was developed based on the data produced by this mechanistic model. This new empirical model was validated against experimental measurements available in the literature for R-134a and R-600a. The new model is fast-running and its performance metrics are in good agreement with experimentally measured data.

An assessment of models for predicting refrigerant characteristics in adiabatic and non-adiabatic capillary tubes

Small vapor compression refrigeration systems incorporate a non-adiabatic capillary tube called a capillary tube-suction line heat exchanger (SLHX) in order to improve performance. The thermodynamic properties of the refrigerant in the capillary tube and suction pipe are influenced by associated phenomena. This study compares various relevant models. Based on the comparison recommended correlations were selected and the simulation results show that the friction factor model has the most dominant.

비단열 모세관 접합방법이 증기압축식 냉동사이클 성능에 미치는 영향 해석

냉동기는 비단열모세관을 채택하여 사용함으로써 사이클의 효율을 향상시킬 수 있다. 비단열모세관은 모세관과 흡입관을 접합함으로써 둘 사이에 열전달이 일어날 수 있도록 한 장치로서 SLHX 라고 부른다. 두 관을 접합하는 방법은 다양하며 이는 사이클의 성능에 영향을 미칠 수 있다. 본 연구에서는 가장 널리 사용되는 두 가지 접합방법이 냉동사이클에 미치는 영향을 해석하였다. 실험결과 용접형 SLHX의 열저항이 테이프형 SLHX의 열저항 보다 크게 작은 것으로 나타났다. 이를 이용한 사이클해석 결과 용접형 SLHX는 COP 와 냉동능력을 5.09%와 14.77% 향상시키고 테이프형 SLHX는 각각 5.05%와 14.75% 향상시켜 둘 사이의 차이는 매우 작은 것으로 나타났다.

Parametric study of a capillary tube-suction line heat exchanger in a transcritical CO 2 heat pump cycle

The capillary tube in a transcritical CO 2 system behaves differently as temperature and pressure are two independent parameters unlike those in a sub-critical cycle. A capillary tube-suction line heat exchanger (CL-SLHX) in a transcritical vapour compression cycle considering homogeneous two-phase flow is modelled in this study based on mass, energy and momentum equations. Effects of gas cooler temperature, evaporator temperature and internal diameter of capillary tube are investigated. Heat transfer rate is observed to be influenced by refrigerant quality, mass flow rate and the prevailing temperature difference. Heat transfer rate variation with gas cooler temperature is unique, recording an initial increase followed by a decrease. Frictional pressure drop influences the heat transfer; consequently, chances of re-condensation of refrigerant vapour are very marginal. Larger diameter of capillary tube leads to increase in refrigerant mass flow rate and increase in heat transfer rate as well. Shorter inlet adiabatic capillary length with larger heat exchanger length is better for heat transfer. This study is an attempt to dispel the scepticism prevailing in transcritical CO 2 system community overemphasising the need for a throttle valve to control the optimum discharge pressure.

Assessment of the energy performance of household refrigerators via dynamic simulation

A first-principles dynamic simulation model was applied to assess the influence of several design parameters – capillary tube-suction line heat exchanger geometry, compressor capacity, internal air flow rate, and temperature range of the thermostat – on the energy performance of a 440-l frost-free refrigerator. The refrigerator performance under faulty conditions, such as condenser fouling, evaporator blockage, insulation aging, and compressor wear, was also investigated. The model was validated against fault-free experimental data showing a reasonable level of agreement for both the start-up and cycling regimes: the system energy consumption was found to be within ±10% agreement with the experimental data, while the compartment air temperatures were predicted with a maximum deviation of ±1 °C.

Modeling of non-adiabatic capillary tube flows: A simplified approach and comprehensive experimental validation

A simplified computational model for simulating refrigerant flow through capillary tubes is proposed and validated using a dataset composed of more than 1400 experimental data points, including adiabatic flows of refrigerants CFC-12, HCFC-22, HFC-134a, HC-600a, R-404A, R-407C and R-507A, and non-adiabatic flows of refrigerants HFC-134a and HC-600a, in both concentric and lateral capillary tube–suction line heat exchanger configurations. The model is based on the mass, energy and momentum conservation equations written according to their one-dimensional differential formulation. Some simplifications were added to the model in order to improve both numerical stability and computational performance. It was found that the model predicts 91.5% of the measured refrigerant mass flow rate for adiabatic and 79.3% for non-adiabatic flows within an error band of ±10%. Also, the model solves non-adiabatic flows as fast as adiabatic ones.

Numerical analysis of refrigerant flow along non-adiabatic capillary tubes using a two-fluid model

This work presents a numerical model to simulate steady state refrigerant flow along capillary tube-suction line heat exchangers, commonly used in small refrigeration systems. The flow along the straight and horizontal capillary tube is divided into two regions: a single-phase and a two-phase flow region. The flow is taken as one-dimensional and the metastable flow phenomenon is neglected. The two-fluid model is employed for the two-phase flow region, considering the hydrodynamic and the thermodynamic non-equilibrium between the liquid and vapor phases. Comparisons are made with experimental measurements of the mass flow rate and temperature distribution along capillary tube-suction line heat exchangers working with refrigerant R134a in different operating conditions. The results indicate that the present model provides a good estimation of the refrigerant mass flow rate. Moreover, comparisons with a homogeneous model are also made. Some computational results referring to the quality, void fraction and velocities of each phase are also presented and discussed.

Numerical simulation of non-adiabatic capillary tubes considering metastable region. Part II: Experimental validation

A detailed one-dimensional steady and transient numerical simulation of the thermal and fluid-dynamic behavior of capillary tube–suction line heat exchangers considering metastable region and separated flow has been developed in Part I of this paper. The developed numerical model allows analysis of aspects such as geometry, type of fluid, critical or non-critical flow conditions and metastable region. The accuracy of the detailed simulation model is demonstrated in this part (Part II) of the paper by comparing simulation results with a wide range of steady state experimental data from the technical literature, which include the refrigerant mass flow rate, outlet suction line temperature, and temperature profile along concentric and lateral capillary tube–suction line heat exchangers. Of the 196 data points evaluated for mass flow rate 96.4% are within an error of ±15%, 81.1% are within ±10% with a mean deviation of ±6.3%. Of the 143 data points evaluated for outlet suction line temperature 89.5% are within an error of ±2 °C, with a mean deviation of ±0.98 °C. The numerical results obtained are used to understand the refrigerant flow behavior inside non-adiabatic capillary tubes. Some divergence problems in the numerical solution process is found to be the discontinuity in non-adiabatic capillary tube flow characteristics caused by re-condensation of the refrigerant within the heat exchanger zone; this aspect needs special attention while modeling the non-adiabatic capillary tube flow. Other important parameter to be evaluated experimentally with special care is the capillary tube internal diameter due to its strong influence on the refrigerant flow results (results of any study based on the nominal diameter are to be used with caution).

Numerical simulation of non-adiabatic capillary tubes considering metastable region. Part I: Mathematical formulation and numerical model

A detailed one-dimensional steady and transient numerical simulation of the thermal and fluid-dynamic behavior of capillary tube–suction line heat exchangers has been carried out. The governing equations (continuity, momentum, energy and entropy) for fluid flows, together with the energy equation in solids, are solved iteratively in a segregated manner. The discretized governing equations in the zones with fluid flow are coupled using a fully implicit step-by-step method. An implicit central difference numerical scheme and a line-by-line solver were used in solids. A special treatment has been implemented in order to consider transitions (subcooled liquid region, metastable liquid region, metastable two-phase region and equilibrium two-phase region). All the flow variables (enthalpies, temperatures, pressures, mass fractions, heat fluxes, etc.) together with the thermophysical and transport properties are evaluated at each point of the grid in which the domain is discretized. The numerical model allows analysis of aspects such as geometry, type of fluid, critical or non-critical flow conditions, metastable regions and transient cases. Comparison of the numerical simulation with experimental data presented in the technical literature will be shown in Part II of the present paper.

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.

Numerical investigation of capillary tube-suction line heat exchanger performance

This paper presents the numerical results of the effect of different parameters on the performance of capillary tube-suction line heat exchangers (CT-SLHX), including condensing and evaporating temperatures, degrees of subcooling and superheat, tube diameter, tube length, and refrigerant flow inlet quality. The heat transfer rate from the capillary tube to the suction line has been simulated for two environmentally friendly refrigerants, namely HFC-134a and HC-600a. The simulation model also postulates the situation where heat may be transferred from capillary tube to the ambient air, before entering the CT-SLHX.

A parametric study of refrigerant flow in non-adiabatic capillary tubes

This paper presents a parametric analysis of refrigerant flow through capillary tube–suction line heat exchangers, used in domestic refrigeration systems. The analysis is based on a homogeneous model developed by the authors. The model is based on the numerical solution of fundamental equations of conservation of mass, momentum and energy of refrigerant flow. The refrigerant flow characteristics are investigated by varying thermodynamic (e.g. condensing temperature, evaporating temperature, inlet sub-cooling, suction line superheat) and geometric parameters (e.g. inlet adiabatic length, heat exchanger length and internal diameter of the capillary tube) of the capillary flow. The source of divergence in the numerical solution process is found to be the discontinuity in non-adiabatic capillary tube flow characteristics caused by re-condensation of the refrigerant within the capillary heat exchanger.

Non-adiabatic capillary tube flow: a homogeneous model and process description

This paper presents a homogeneous model of refrigerant flow through capillary tube–suction line heat exchangers, which are widely used in small vapour compression refrigeration systems. The homogeneous model is based on fundamental conservation equations of mass, momentum and energy. These equations are solved simultaneously through iterative process. Churchill’s correlation [3] is used to calculate single-phase friction factors and Lin et al. [6] correlation for two-phase friction factors. The single-phase heat transfer coefficient is calculated by Gnielinski’s equation [5] while two-phase flow heat transfer coefficient is assumed to be infinite. The model is validated with previous experimental and analytical results. The present model can be used in either design calculation (calculate the capillary tube length for given refrigerant mass flow rate) or simulation calculation (calculate the refrigerant mass flow rate for given capillary tube length). The simulation model is used to understand the refrigerant flow behaviour inside the non-adiabatic capillary tubes.