Showing papers in "International Journal of Refrigeration-revue Internationale Du Froid in 2006"

A computationally effective formulation of the thermodynamic properties of LiBr-H2O solutions from 273 to 500 K over full composition range

Abstract: A set of computationally efficient formulations of thermodynamic properties of LiBr–H 2 O solutions at vapor–liquid equilibrium is presented in the form of explicit separate functions of temperature and mixture composition. The set includes pressure, density, isobaric heat capacity, enthalpy, and entropy. The description of these properties is valid from 273 K or from the crystallization line up to 500 K in temperatures and for mixture composition from 0 to 75 wt% of LiBr in the solution. The equations are based upon a body of experimental data that have been critically assessed. Gaps in the database are shown to give experimenters orientation for future research. The structure of the equations guarantees an explicit transition to the properties of pure water for lithium bromide concentration approaching zero. The uncertainties associated with correlation are estimated to be ±0.5% for density, ±2.1% for pressure and ±2% for isobaric heat capacity. The uncertainty in values of enthalpy is estimated to be ±10 kJ kg −1 and ±0.03 kJ kg −1 K −1 for entropy. Values of the particular properties generated by the representative equations are provided to assist with the confirmation of computer implementation of the calculation procedure.

Printed circuit heat exchanger thermal–hydraulic performance in supercritical CO2 experimental loop

Abstract: Heat transfer and pressure drop characteristics of the Printed Circuit Heat Exchanger (PCHE) were investigated in an experimental supercritical CO2 loop. The inlet temperature and pressure were varied from 280 to 300 °C/2.2 to 3.2 MPa in the hot side and from 90 to 108 °C/6.5 to 10.5 MPa in the cold side while the mass flow rate was varied from 40 to 80 kg h−1. The overall heat transfer coefficient range is 300–650 W m−2 K−1 while the compactness with respect to the heat exchanger core is approximately 1050 m2 m−3. The empirical correlations to predict the local heat transfer coefficient and pressure drop factor as a function of the Reynolds number have been proposed for the tested PCHE.

Design and performance of a permanent-magnet rotary refrigerator

Abstract: In order to demonstrate the potential of magnetic refrigeration to provide useful cooling near room temperature, Astronautics Corporation of America constructed a rotary magnetic refrigerator (RMR) in 2001. The RMR uses the active magnetic regenerator (AMR) cycle with an aqueous heat transfer fluid. The required change in magnetic field is produced by the rotation of a wheel packed with porous beds of magnetocaloric material through a 1.5 T Nd2Fe14B permanent magnet with steel flux concentration poles. A pump, and valves mounted to the wheel, control heat transfer fluid flow through the magnetocaloric beds and heat exchangers. This rotary design allows quiet, reliable operation over a range of frequencies (0.5–4 Hz), heat transfer fluid flow rates and cooling power. The performance of the device using Gd and Gd alloy spherical particles is reported and analyzed. We also describe the performance effects of introducing layered beds and an La(Fe1−xSix)13Hy alloy with a first order magnetic transition.

Advanced magnetocaloric materials: What does the future hold?

Abstract: Recent achievements in the design of robust near room temperature magnetic cooling devices signify paradigm shift in refrigeration, liquefaction and freezing technologies, and call for a much broader base of advanced magnetocaloric materials to support quick materialization of this environmentally friendly, energy efficient technology in a variety of markets. The latest material discoveries are reviewed and current trends in engineering of advanced magnetocaloric compounds have been identified.

Comparison of CFD analysis to empirical data in a commercial vortex tube

Abstract: This paper presents a comparison between the performance predicted by a computational fluid dynamic (CFD) model and experimental measurements taken using a commercially available vortex tube. Specifically, the measured exit temperatures into and out of the vortex tube are compared with the CFD model. The data and the model are both verified using global mass and energy balances. The CFD model is a two-dimensional (2D) steady axisymmetric model (with swirl) that utilizes both the standard and renormalization group (RNG) k-epsilon turbulence models. While CFD has been used previously to understand the fluid behavior internal to the vortex tube, it has not been applied as a predictive model of the vortex tube in order to develop a design tool that can be used with confidence over a range of operating conditions and geometries. The objective of this paper is the demonstration of the successful use of CFD in this regard, thereby providing a powerful tool that can be used to optimize vortex tube design as well as assess its utility in the context of new applications.

Thermodynamic analysis of optimal condensing temperature of cascade-condenser in CO2/NH3 cascade refrigeration systems

Abstract: This study thermodynamically analyzed a cascade refrigeration system that uses carbon dioxide and ammonia as refrigerants, to determine the optimal condensing temperature of the cascade-condenser given various design parameters, to maximize the COP and minimize the exergy destruction of the system. The design parameters include: the evaporating temperature, the condensing temperature and the temperature difference in the cascade-condenser. The results agreed closely with the reported experimental data. The optimal condensing temperature of the cascade-condenser increases with TC, TE and ΔT. The maximum COP increases with TE, but decreases as TC or ΔT increases. Two useful correlations that yield the optimal condensing temperature of the cascade-condenser and the corresponding maximum COP are presented.

CoilDesigner: a general-purpose simulation and design tool for air-to-refrigerant heat exchangers

Abstract: A simulation and design tool to improve effectiveness and efficiency in design, and analysis of air to refrigerant heat exchangers, CoilDesigner, is introduced. A network viewpoint was adopted to establish the general-purpose solver and allow for analysis of arbitrary tube circuitry and mal-distribution of fluid flow inside the tube circuits. A segment-by-segment approach within each tube was implemented, to account for two-dimensional non-uniformity of air distribution across the heat exchanger, and heterogeneous refrigerant flow patterns through a tube. Coupled heat exchangers with multiple fluids inside different subsets of tubes can be modeled and analyzed simultaneously. A further sub-dividing-segment model was developed in order to address the significant change of properties and heat transfer coefficients in the single-phase and two-phase regime when a segment experiences flow regime change. Object-oriented programming techniques were applied in developing the program to facilitate a modular, highly flexible and customizable design platform and in building a graphic user-friendly interface. A wide variety of working fluids and correlations of heat transfer and pressure drop are available at the user's choice. The model prediction with CoilDesigner was verified against experimentally determined data collected from a number of sources.

The effect of nano-particles on the bubble absorption performance in a binary nanofluid

Abstract: The objectives of this paper are to examine the effect of nano particles on the bubble type absorption by experiment and to find the optimal conditions to design highly effective compact absorber for NH3/H2O absorption system. The initial concentrations of NH3/H2O solution and the kinds and the concentrations of nano particles are considered as key parameters. The results show that the addition of nano particles enhances the absorption performance up to 3.21 times. Moreover, the absorption rate increases with increasing concentration of nano particles and the nano particles are more effective for lower absorption potential solution. The potential enhancement mechanism for binary nanofluid is suggested. The experimental correlations of the effective absorption ratio for each nano particles, Cu, CuO, and Al2O3, are suggested within ±10% error-band.

Modelling of food transportation systems - a review

Abstract: In 2002, over a million refrigerated road vehicles, 400,000 refrigerated containers and many thousands of other forms of refrigerated transport systems are used to distribute chilled and frozen foods throughout the world. All these transportation systems are expected to maintain the temperature of the food within close limits to ensure its optimum safety and high quality shelf life. Increasingly, modelling is being used to aid the design and optimisation of food refrigeration systems. Much of this effort has concentrated on the modelling of refrigeration processes that change the temperature of the food such as chilling, freezing and thawing. The purpose of a refrigerated transport system is to maintain the temperature of the food and appears to have attracted less attention from modellers. This paper reviews the work that has been carried out specifically on the modelling of food temperature, microbial growth and other parameters in the transportation of food. © 2006 Elsevier Ltd and IIR.

A review of numerical models of airflow in refrigerated food applications

Abstract: Temperature homogeneity in most food refrigeration systems is directly governed by the airflow patterns in the system. Numerical modelling of airflow provides an opportunity to develop improved understanding of the underlying phenomena influencing system performance, which can lead to reduced temperature heterogeneity and increased effectiveness and efficiency of refrigeration systems. With the rapid advances in computational power of recent years, the use of Computational Fluid Dynamics (CFD) techniques in this application has become popular. This paper reviews the application of CFD and other numerical modelling techniques to the prediction of airflow in refrigerated food applications including cool stores, transport equipments and retail display cabinets.

The primary chilling of poultry carcasses—a review

Abstract: This paper reviews the published scientific studies that have been carried out on the chilling of poultry carcasses. The prime purpose of chilling is to limit the growth of both pathogenic and food spoilage microorganisms. There are a wide range of publications that show that, in general, the numbers of both types of microorganism, on the surface of poultry carcasses, is reduced during the chilling process. Immersion or spin chilling is not used in the production of ‘fresh’ chilled poultry in Europe, ‘dry’ air chilling being the preferred chilling method. Many people believe that there is some clear microbiologically based reason behind the selection of air chilling. However, the published data do not appear to support this belief, and if anything point to a microbial advantage of immersion systems. The rate of chilling has some influence on the taste, texture and appearance of poultry meat. Very rapid chilling can result in tougher chicken meat, while very slow chilling can produce pale soft exudative (PSE) muscle. However, in both cases the effect is not as marked as with red meat. No comprehensive published data has been found on the effect of a range of chilling systems, chilling conditions, carcass weight and shape on the rate of chilling, weight loss and heat loss. Without such data it is impossible to optimise the design of a poultry chilling system.

Thermodynamics of magnetic refrigeration

Abstract: A comprehensive treatment of the thermodynamics of cyclic magnetic refrigeration processes is presented. It starts with a review of the work, heat and internal energy of a magnetized specimen in a magnetic field, and a list of the thermodynamic potentials is given. These are based on the very recent discovery of an alternative Kelvin force. It is shown that this force is compatible with the internal energy proposed by Landau and Lifshitz. New formulas for the specific enthalpies are presented. Cyclic processes are discussed in detail, e.g. the Brayton, Ericsson and Carnot cycles. Magnetic refrigeration and magnetic heat pump cycles are preferably designed by applying the cascade or/and regeneration principle. Cascade systems allow wider temperature ranges to be obtained. The main objective of this article is to yield a theoretical basis for an optimal design of new magnetic refrigeration and heat pump devices.

Experimental investigation of a three-material layered active magnetic regenerator

Abstract: This work reports on experimental studies using an active magnetic regenerative test apparatus (AMRTA) in near room-temperature refrigeration cycles Experiments using regenerator beds composed of three different magnetocaloric materials combined in a layered configuration with applied fields of 2 T have produced no-load temperature spans in excess of 50 K The test apparatus uses two active magnetic regenerators each containing approximately 135 g of refrigerant An overview of the test apparatus, operating parameters, and performance is described The impacts of operation at varying heat rejection temperatures, applied fields of 15 T and frequencies between 065 and 10 Hz are presented In addition, the impacts of operating pressure and applied load on temperature spans are discussed

Experimental investigation on R134a vapour ejector refrigeration system

Abstract: The experimental investigation of the performance of a vapour ejector refrigeration system is described. The system uses R134a as working fluid and has a rated cooling capacity of 0.5 kW. The influence of generator, evaporator and condenser temperatures on the system performance is studied. This kind of system can be operated with low grade thermal energy such as solar energy, waste heat, etc. The operating conditions are chosen accordingly as, generator temperature between 338 K and 363 K, condenser temperature between 299 K and 310.5 K, and evaporator temperature between 275 K and 285.5 K. Six configurations of ejectors of different geometrical dimensions are selected for the parametric study. The performance of the refrigeration system at different operating temperatures is presented.

Modelling transport phenomena in refrigerated food bulks, packages and stacks: basics and advances

Abstract: Transport phenomena, comprising airflow, heat and mass transfer, are key processes in refrigerated storage. This paper gives a review of approaches to modelling transport phenomena in food bulks, packages and stacks. Darcy–Forchheimer porous media models have been successfully used. Ergun theory was found not directly applicable when foods are stored in packages, which cause flow confinement and vent hole resistance, invalidating traditional theory. Heat and mass transfer in food bulks has been modelled using single- and two-phase models. Suggested modifications were demonstrated to not be generally valid, leaving much scope for further development of models for refrigerated system design. Direct CFD approaches were shown to be successful alternatives to achieve this goal.

Simulation of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applicationsElimination des cristaux de givre sur une plaque froide: effets des fréquences stationnaires et de balayage des champs électriques

Abstract: A steady state simulation model has been developed to evaluate the system performance of a transcritical carbon dioxide heat pump for simultaneous heating and cooling. The simulated results are found to be in reasonable agreement with experimental results reported in the literature. Such a system is suitable, for example, in dairy plants where simultaneous cooling at 4 °C and heating at 73 °C are required. The optimal COP was found to be a function of the compressor speed, the coolant inlet temperature to the evaporator and inlet temperature of the fluid to be heated in the gas cooler and compressor discharge pressure. An optimizing study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on the heat exchanger area has been carried out. Effect of heat transfer in the heat exchangers on system performance has been presented as well. Finally, a novel nomogram has been developed and it is expected to offer useful guidelines for system design and its optimisation.

Modelling heat and mass transfer in frozen foods: a review

Abstract: This paper reviews mathematical methods for modelling heat and mass transfer during the freezing, thawing and frozen storage of foods. It starts by considering the problems in modelling heat transfer controlled freezing (the Stefan problem): release of latent heat, sudden changes in thermal conductivity. The author gives a unified overview of the common numerical methods: finite difference, finite element and finite volume. Mass transfer is then considered, involving different phenomena and approaches for dense and porous foods. Supercooling, nucleation and trans-membrane diffusion effects during freezing, and recrystallization during frozen storage are considered next. High pressure thawing and thawing are considered in view of their recent popularity. Finally, the paper offers a brief look at mechanical stresses during freezing, a much neglected area. It is concluded that while modelling heat transferred controlled freezing is a settled problem, much work remains to be done in modelling associated phenomena in order to gain the ability to predict changes in food quality at the micro-level.

An experimental study on heat transfer and pressure drop characteristics of carbon dioxide during gas cooling process in a horizontal tube

Abstract: The heat transfer coefficient and pressure drop during gas cooling process of CO₂ (R744) in a helically coiled copper tube with the inner diameter of 4.55 ㎜ and outer diameter of 6.35 ㎜ were investigated experimentally. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and a helically coiled type gas cooler (test section). The refrigerant mass fluxes are varied from 200 to 800 ㎏/㎡s and the inlet pressures of gas cooler are 7.5 to 10.0 ㎫. The heat transfer coefficients of CO₂ in a helically coiled tube are higher than those in a horizontal tube. The pressure drop of CO₂ in the gas cooler shows a relatively good agreement with those predicted by Ito’s correlation developed for single-phase in a helically coiled tube. The local heat transfer coefficient of CO₂ agrees well with the correlation by Pitla et al. However, at the region near pseudo-critical temperature, the experiments indicate higher values than the Pitla et al. correlation. Therefore, various experiments in helically coiled tubes have to be conducted and it is necessary to develop the reliable and accurate prediction determining the heat transfer and pressure drop of CO₂ in a helically coiled tube.

Frost, defrost, and refrost and its impact on the air-side thermal-hydraulic performance of louvered-fin, flat-tube heat exchangers

Abstract: The thermal-hydraulic performance under conditions of an initial frost growth on the air-side surface, and for subsequent ‘refrosting’ after a defrost period is experimentally studied for folded-louvered-fin, microchannel heat exchangers In total, five heat exchangers are considered; the thermal performances during one frost-growth cycle for four different fin geometries are compared in terms of overall heat transfer coefficient, pressure drop, and j and f factors; the defrost and refrost characteristics of two heat exchangers are compared to explore geometry effects Typically, the performance under refrosting conditions becomes periodic and repeatable after the third or fourth refrosting cycle The allowable frost growth period (before a defrost is required), the defrost requirement, and the thermal-hydraulic performance depend on heat exchanger geometry for the specimens used in this study

Performance of a room-temperature rotary magnetic refrigerator

Abstract: We have designed and operated a rotating-magnet type AMR (active magnetic regeneration) refrigerator that uses water as a heat transfer fluid. Four kinds of gadolinium-based alloy are used as magnetic materials. A magnetic field of 0.77 T is applied by neodymium permanent magnets. The refrigerator produces a maximum cooling power of 60 W around 10 °C. An optimal time for one cycle exists, and it depends on the water flow rate and the frequency of magnetization and demagnetization. Enhancement of the water flow rate and the frequency is known to be essential for increasing the cooling power of this refrigerator.

Thermoeconomic evaluation and optimization of an aqua-ammonia vapour-absorption refrigeration system

Abstract: In this paper, the thermoeconomic concept is applied to the optimization of an aqua-ammonia vapour-absorption refrigeration (VAR) system—aimed at minimizing its overall product cost. The thermoeconomic concept based simplified cost minimization methodology calculates the economic costs of all the internal flows and products of the system by formulating thermoeconomic cost balances. The system is then thermoeconomically evaluated to identify the effects of design variables on costs and thereby enables to suggest values of design variables that would make the overall system cost-effective. Based on these suggestions, the optimization of the system is carried out through an iterative procedure. The results show a significant improvement in the system performance without any additional investment. Finally, sensitivity analysis is carried out to study the effect of the changes in fuel cost to the system parameters.

Evaporative heat transfer and pressure drop of R410A in microchannels

Abstract: Convective boiling heat transfer coefficients and two-phase pressure drops of R410A are investigated in rectangular microchannels whose hydraulic diameters are 1.36 and 1.44 mm. The mass flux was varied from 200 to 400 kg/m2s, heat flux from 10 to 20 kW/m2, as the saturation temperatures were maintained at 0, 5 and 10 °C. A direct heating method was used to provide heat flux into the fluid. The boiling heat transfer coefficients of R410A in the microchannels were much different with those in single tubes, and the test conditions only slightly affected the heat transfer coefficients before dryout vapor quality. The present heat transfer correlation for microchannels, which was developed by introducing non-dimensional parameters of Bo, Wel, and Rel used in the existing heat transfer correlations for large diameter tubes, yielded satisfactory predictions of the present data with a mean deviation of 18%. The pressure drops of R410A in the microchannels showed very similar trends with those in large diameter tubes. The existing two-phase pressure drop correlations for R410A in microchannels satisfactorily predicted the present data.

Review of effective thermal conductivity models for foods

Abstract: The literature associated with modelling and predicting the thermal conductivities of food products has been reviewed. The uncertainty involved in thermal conductivity prediction increases as the differences between the food components' thermal conductivities increase, which means that there is greater uncertainty involved with predicting the thermal conductivity of foods which are porous and/or frozen, than with unfrozen, non-porous foods. For unfrozen, non-porous foods, a number of simple effective thermal conductivity models that are functions only of the components' thermal conductivities and volume fractions may be used to provide predictions to within ±10%. For frozen and/or porous foods, the prediction procedure is more complicated, and usually requires the prediction of porosity and/or ice fraction, which introduces another source of error. The effective thermal conductivity model for these foods may require an extra parameter (in addition to the components' thermal conductivities and volume fractions) whose value must often be determined empirically. Recommendations for selecting models for different classes of foods are provided. There is scope for more research to be done in this area.

A generalized moving-boundary model for transient simulation of dry-expansion evaporators under larger disturbances

Abstract: A generalized model based on the moving-boundary approach is developed to describe the transient behavior of dry-expansion evaporators in the vapor-compression refrigeration system. To improve the robustness of the traditional moving-boundary model under larger disturbances, the time-variant mean void fraction is employed instead of the constant. Numerical integration is applied to get the mean properties in the two-phase region and the superheated region. The interface wall temperature between the two-phase and the superheated regions is also evaluated by a new weighted mean. Qualitative case study shows that the present model can well predict the transient behaviors of evaporators under larger disturbances and keep the robustness whenever superheated region appears or disappears.

Effectiveness and optimum jet velocity for a plane jet air curtain used to restrict cold room infiltration

Abstract: The effectiveness of a 1.0 m wide air curtain fitted over a 1.36 m wide entrance to a cold store has been evaluated. It was shown that careful setting up of the air curtain (adjusting the jet velocity and angle) was needed, this achieved an effectiveness of 0.77 compared to the initial value of only 0.31 as set by the installer. An analytical model to predict the optimum jet velocity was compared to measured data. It is important to choose the correct safety factor (an increase in the jet velocity) for this model, as an effectiveness of between 0.37 and 0.70 could be produced using the range of safety factors found in the literature. A 2D computational fluid dynamics (CFD) model predicted a maximum effectiveness of 0.84 and showed how the effectiveness of the curtain was related to the shape of the jet and how this varied with jet velocity and door open duration.

Composite adsorbent of CaCl2 and expanded graphite for adsorption ice maker on fishing boats

Abstract: Adsorption performances and thermal conductivity were tested for three types of adsorbent: Pure CaCl2 powder, simple composite adsorbent and consolidated composite adsorbent. The simple composite adsorbents show better adsorption performance because the additive of expanded graphite in CaCl2 powder has restrained the agglomeration phenomenon in adsorption process and improved the adsorption performance of CaCl2. The consolidated composite adsorbent are suitable to be used as adsorbent for ice maker on fishing boats because they have higher thermal conductivity, larger volumetric cooling capacity, higher SCP values and better anti-sway performance than simple composite adsorbents. Thermal conductivity of the consolidated composite adsorbent is 6.5–9.8 W m−1 K−1 depending on the molding pressure, ranging from 5 to 15 MPa, which is about 32 times higher than the thermal conductivity of CaCl2 powder. The volumetric cooling capacity of consolidated composite adsorbent is about 52% higher than the best result obtained for CaCl2 at the evaporating temperature of −10 °C. The SCP of the consolidated adsorbent increases of about 353% than CaCl2 powder from simulation results at Tad=30 °C and Tev=−10 °C. The consolidated composite adsorbents have good anti-sway performance and they are not easy to be scattered out when the fishing boats sway on the sea.

Momentum transfer of ice slurry flows in tubes, experimental investigations

Abstract: In this paper we present results of the studies of ice slurry flow in horizontal tubes. The possibility of treating the rheological parameters of ice slurry as being those of Bingham fluid was confirmed. The values of parameters (mass fraction, flow velocity) corresponding to the laminar, intermediate and turbulent flow were determined which permits to optimize the flow in the systems working with this cooling agent. Critical flow velocity and mass fraction of ice values were determined thereby; they correspond to a change in character of an ice slurry flow from a laminar to turbulent motion. Experimental results were compared to the analytical results, based on the Hedstrom and Tomita algorithms (the laminar and turbulent flow, respectively). The comparison showed a very good agreement between these data.

The application of photo-coupler for frost detecting in an air-source heat pump

Abstract: This experimental study is carried out to investigate reliability and effectiveness of a new method of using photo-coupler for detecting frost formation in an air source heat pump, and further to determine the most efficient initiation point of the defrost cycle. This new method of using photo-coupler as a frost sensing device is evaluated by comparing its performance with conventional time control defrost system in which defrost cycle is set to start at predetermined interval, e.g. about at every 1–1.5 h. Results indicate that overall heating capacity of photo-coupler detection method (case IV) is 5.5% higher than that of time control method. It is also shown that for maximum efficiency the defrost cycle must be initiated before the frost build-up area exceeds 45% of total front surface of the outdoor coil.

A study of accurate latent heat measurement for a PCM with a low melting temperature using T-history method

Abstract: When the latent heat of a phase change material (PCM) with a lower melting point than ambient temperature was assessed according to the standard T-history method using a vertically oriented test tube, a temperature gradient occurred in the longitudinal direction of the tube due to natural convection. This led to a decrease in the accuracy of the latent heat of fusion measurement. In this study, the accuracy of the measurement with the original T-history method was improved without decreasing the test's simplicity and convenience by setting the test tube horizontally. The heat transfer to the vapor-layer of the tube under volume change during melting was assumed to be negligible and the results were calculated using the two inflection points of temperature as the start and end of latent heat period. Under these assumptions, the results agree closely with other reference data. And, the new method proposed in this study showed a remarkable reduction in data scattering.

Potential for cost effective magnetocaloric air conditioning systems

Abstract: Magnetic refrigeration is an emerging technology that exploits the magnetocaloric effect found in solid-state refrigerants. The combination of solid-state refrigerants, water-based heat transfer fluids, and high efficiency will lead to environmentally desirable products with minimal contributions to global warming. Among the numerous applications of refrigeration technology, air conditioning applications provide the largest aggregate cooling power and use the greatest quantity of electric energy. The primacy of the air conditioning application makes the establishment of cost targets for this application an essential feature of the R&D plan for magnetic refrigeration technology. A preliminary assessment of the permanent magnet costs and magnetocaloric material costs indicates that, for suitably chosen materials and operating conditions, these costs lay well below the total manufactured costs for vapor compression based air conditioners.