Oil Cooler Research Articles

Enhanced prediction of the decongealing performance of aero-engine oil coolers with various fin types and operating conditions using lumped model

Freezing of the flight environment of an aircraft occasionally causes the congealing of oil in the oil coolers. Although decongealing tests and re-designs require longer durations, many studies have examined the cooling loss arising from this phenomenon. Numerical simulations for the decongealing process are computationally expensive, and experiments under typical operating scenarios are costly. In this paper, an efficient one-dimensional flow and thermal approach is proposed, with transient simulations under realistic decongealing scenarios involving a standard fuel cooled oil cooler. With the objective function being the maximization of the heat transfer and minimization of the pressure drop, different cooler configurations with various internal fins were examined for both design and off-design conditions. The proposed lumped model provides a similar accuracy with a lower calculation cost compared to an earlier model. It also ensures better stability and easier code modifications. The current study demonstrates an optimum heat transfer and pressure drop with improved total decongealing performance when offset-strip fins on both the oil and fuel sides (ΔP: 23.9 psi, Tout: 12.3 °C) or an offset-strip fin on the oil side and a wavy or pin fin on the fuel sides (ΔP: 34.2 psi, Tout: 11.4 °C) are applied.

Modified air-Fenton with MIL-88A for chemical oxygen demand treatment in used coolant oil.

Coolant oil from auto part manufacturing contains additives resulting in high chemical oxygen demand (COD) in wastewater. In this study, COD treatment of coolant oil was investigated in a metal-organic framework (MOF) with MIL-88A by a modified air-Fenton (MAF) process by varying synthetic coolant oil concentrations (1-5%), pH (3-9), air-flow rate (1-2 L/min), amount of MIL-88A (0.2-1.0g), and reaction time (30-180min). The results were analyzed using central composite design (CCD) and response surface methodology (RSM) using Minitab ver. 19. The characteristic MIL-88A was characterized by XRD that showed a spindle-like shape with 2θ at 10.2° and 13.0°. The FTIR spectrum revealed the vibrational frequencies at Fe-O (564cm-1), C-O (1391 and 1600cm-1), and C = O (1216 and 1710cm-1). The optimum treatment efficiency was studied from 30 CCD conditions in the presence of coolant oil (5%, COD ~ 132,000mg/L), pH (9), air flow rate (2 L/min), and MIL-88A (1g) within 177min. The results obtained from the experiment and the COD prediction were found to be 92.64% and 93.45%, respectively. The main mechanism of iron(III) in MIL-88A is proposed to be the production of hydroxyl radical (·OH) that oxidizes the organic matter in the coolant oil. Moreover, the MAF process was applied to the used industrial coolant oil and was found to be 62.59% efficient.

Simulation of shell and tube type heat exchanger for square geometry using computational fluid dynamics method

Heat exchangers have various types of design models, one example is the shell and tube type heat exchanger. This tool is useful for raising or lowering the temperature. One of its uses is in the hydroelectric power generation industry, namely as an oil cooler bearing. This study aims to design and simulate a shell and tube type heat exchanger with tube inner diameters of 20 mm and 25 mm and also tube arrangement square and rotated square. From the design results obtained a total length of 1318.59 mm, 300 mm outer diameter of the shell and 320 mm inner diameter of the shell. The simulation results show that variations in tube diameter and tube arrangement greatly affect the performance of the heat exchanger, the highest value for the overall heat transfer coefficient is found in the tube square arrangement variation with the number of tubes 61 of 112.15 W/m2.K and the lowest value in the tube arrangement variation square with the number of tubes 55 is 106.48 W/m2.K, the highest value of pressure drop on the shell side is found in the variation of the tube square arrangement with the number of tubes 55.

Identification of the effect of strontium and ytterbium addition over magnesium zinc alloy during the drilling process with ARAS and WASPAS techniques

Magnesium (Mg) alloys are widely employed in aerospace and automotive applications owing to their beneficial attributes. Identifying proper process parameters is a critical choice to save costs and increase quality. This necessitates the classification of suitable process parameters in light of numerous competing considerations; hence it must be analyzed by means of the Multiple Criteria Decision Making (MCDM) techniques. This paper handles the MCDM issue using two different approaches namely, the ARAS (Additive Ratio ASsessment) and WASPAS (Weighted Aggregated Sum Product ASsessment). The experiments carried out in Mg Zn10 and adding of Strontium (Sr) and Ytterbium (Yb) were critically evaluated. The independent variable like spindle speed and feed rate were examined by varying the machining tool and coolant supplied. The effect of coated and uncoated carbide was studied with High-Speed Steel. The influence of coconut oil, liquid nitrogen, and vegetable oil over the process parameters was also studied. The responses like thrust force, torque, and surface roughness were studied over the three different materials by varying the numerical and categorical factors. The addition of Yb plays a beneficial effect on all three responses considered. The ideal combination was discovered using ARAS and WASPAS for concomitant reduction of all the considered responses, attained with a coated carbide tool with a spindle speed of 118 m/min. and a feed rate of 0.26 mm/rev. during coconut oil coolant supply over Mg Zn10 Sr2. The ARAS rankings align with the WASPAS rankings in 55.56% of cases, with the interesting observation that both the lower and higher orders are identical. The optimal process parameters for Mg Zn10 were identified as a cutting speed of 56 m/min. and a feed rate of 0.26 mm/rev., using a coated carbide tool and CO coolant. For Mg Zn10 Sr2 Yb2, the study found the optimal parameters to be a cutting speed of 118 m/min. and a feed rate of 0.62 mm/rev., with the use of uncoated carbide and CO coolant.

Optimization of Lube Oil Cooler Fan at Reserve Shutdown Combined Cycle Power Plant Priok

Optimization of Lube Oil Cooler Fan at Reserve Shutdown Combined Cycle Power Plant Priok

Research on influence of grain boundaries on the mechanical properties of 460 MPa refractory steel used for high-strength building structures

In this paper, the microstructure of Mo-type seismic refractory steel for building g, as well as different boundary densities and boundary ratios, are combined with elevated-temperature mechanical properties analysis to explore the laws of boundary for high-temperature performance. The results show that salt water cooling (SWC) and water cooling (WC) can obtain lath bainite with a higher content, and oil cooling (OC) with a lower cooling rate can obtain the microstructure of multiphase bainite + bulk ferrite. The boundary characterization results show that when the sample contains more high angle grain boundaries (Block and High-Packet boundaries), and the dislocation density is high, it can make it have better mechanical properties at room temperature. When the content of low angle boundary and low interfacial energy twin boundary (Σ3 boundary, which is mainly composed of V1/V2 variant pair) is high, it will have better microstructure stability after high temperature tempering, and the boundary density and dislocation density will decrease slightly, ensuring that it has better refractory performance.

ДОСЛІДЖЕННЯ ЕФЕКТИВНОСТІ ТЕПЛООБМІННОГО ПРОЦЕСУ В ОХОЛОДЖУВАЧІ ТУРБІННОГО МАСТИЛА МО-2 В ЗАЛЕЖНОСТІ ВІД СХЕМИ РОЗТАШУВАННЯ ТРУБОК У ТРУБНИХ ҐРАТАХ

Effective cooling of the turbine oil is important for the reliability and optimal operation of the turbogenerator. The oil is used for lubrication and cooling of the support and radial bearings of the turbine, bearings of the generator and reducer, gears and splined rollers. It is also used as a hydraulic fluid in the hydraulic turbine start-up system. For reliable and safe operation of the system, it is necessary to maintain the correct temperature of the lubricant. Research is aimed at studying the influence of the layout of the heat exchange tubes in the tube grids on the cooling efficiency of the TP-22 turbine oil. With the help of the SolidWorks software complex, a geometric model of the MO-2 turbine oil cooler was created with three schemes for the placement of heat exchange tubes in tube grids: corridor, radial and checkerboard. Adhering to the parameters of the technical characteristics of this cooler, the nominal parameters of temperatures and consumption of coolants (cooling water and turbine lubricant TP-22) were chosen. The result of the calculations were the temperature fields of the coolants and the trajectories of the movement of lubricant particles in the intertube space of the cooler. When creating a geometric model and further research, the main technical characteristics of the shell-and-tube horizontal cooler MO2 were adopted, namely: the temperature of the turbine oil at the inlet is 45°С, the nominal flow rate of the oil is 3.1 m3/h, the temperature of the water at the inlet is 20°С, the nominal flow rate water - 15 m3/h. The conducted computer modeling made it possible to investigate the influence of the layout of the heat exchange tubes in the tube grids on the cooling efficiency of the turbine lubricant in the MO-2 shell-and-tube cooler. It was shown that the most effective, among the studied schemes, is the checkerboard arrangement of heat exchange tubes in tube grids. This scheme makes it possible to reduce the consumption of cooling water, relative to the nominal one, which will allow the use of water pumps of lower power.

On design and analysis of a concentric tube micro fin heat exchanger for applications of tire pyrolysis lube oil cooling

Concentric tube heat exchangers are widely used as lube oil coolers in large internal combustion engines. They are particularly preferred because of simple manufacturing, lower area, and better heat transfer characteristics. With the growing use of unconventional fuel-based lubricants, their design is continuously improved to achieve the required heat transfer characteristics. One of the emerging alternative oil is Tire Pyrolysis Oil (TPO) to minimize tire waste and achieve the waste-to-energy objective. The TPO is employed not only as diesel blended fuel but also as a lube oil because of its satisfactory tribological properties. The present work provides a detailed design and analysis of a concentric-tube micro-fin-based lube oil cooler for TPO applications. The design encompasses the thermal, hydraulic, and economic characteristics of the heat exchanger subjected to TPO, Ethylene Glycol (EG), and Crude Oil (CO). To achieve this a robust numerical model is developed and implemented using engineering equation solver software. The model is validated with experimental data first. Then a detailed parametric analysis is done to investigate the heat exchanger performance. It was observed that TPO offered a better heat transfer coefficient along with lesser power input and cost compared to EG and CO. For instance, in the case of a mass flow rate 0.1 kg/s, the heat transfer per unit pressure drop is highest for TPO with 5.86 m/sK followed by CO with 5.12 m/sK and EG with 0.3876 m/sK. Similarly, the annual operational cost is calculated as 27.7 k$, 31.4 k$, and ∼ 80.8 k$ for the TPO, CO, EG which shows the superiority of TPO as lube oil for engine applications. Based on thermal, hydraulic design and economic analysis, it is concluded that tire pyrolysis oil (TPO) is used in the concentric tube micro fin heat exchanger for applications of oil cooling.

Experimental investigation of an automobile radiator using carbon based hybrid nano coolant

Comprehensive experimentation on the heat transfer performance of a vehicle radiator with a novel hybrid nanofluid (HyNf) comprising Graphene nanoplatelets (GnP) and Multi-walled carbon nanotubes (MWCNT) nanoparticles in 1:1 ratio, dispersed in radiator coolant oil (base fluid), has been performed. Non-covalent functionalization method was implemented to prepare steady nanofluids for volume concentrations of 0.05%, 0.1%, 0.15%, 0.2%, 0.3%, 0.4%, and 0.5%. Tests were conducted for different nanofluid flow rates from 20 g/s to 100 g/s in 5 steps, three different nanofluid entry temperatures of 30 °C, 50 °C and 70 °C and three different air velocities viz. 1, 3 and 5 m/s. The enhancement in thermal conductivity (TC) was observed as 59% for 0.5 vol % compared to base fluid, which leads to an augmentation in the convective heat transfer coefficient (CHTC) of nanofluid. The maximum improvement in overall heat transfer coefficient (OHTC) was found as ∼ 124% at 0.5% volume concentration, 5 m/s air velocity, compared to the base fluid. A rise in the pressure drop is found as 23.72% compared to base fluid. Due to these enhanced heat transfer behaviours of the synthesized GnP-MWCNT/coolant oil HyNf, it can be an alternate to the existing coolant oil.

Modification of The Oil Cooler to Avoid Oil Leaking in Generator Transformer PLTU Labuan “Moderator”

The Main Generator Transformer for PLTU Labuan is a transformer with a power capacity of 350 MVA and a cooling system of 6 pcs OFAF (Oil Force Air Force) type. During operation, a disturbance to the Main Generator Transformer (GT) was caused by an oil cooler leak. This is caused by the corrosive expand tube caused by the constituent material consisting of 2 different types of materials. The oil leak caused environmental pollution, decreased oil level, and increased winding temperature and oil temperature on GT. During the decade of operation of the PLTU Labuan, GT oil cooler leaks almost always occur every year, both in unit 1 and unit 2. To anticipate this, we made modifications to the end tube design by changing the type of material on the expanded tube from initially using iron to aluminum such as tube material outside the expanded position. The next design change is to increase the length of the end tube which is equipped with the knock and glue with an expansion strength above normal operating temperature which makes the end tube more reliable by increasing the surface area attached to the body oil cooler. In addition to making changes to the design, this modification reuses used oil coolers that have good tube quality so that they can be reused with more reliable quality. By making these modifications, it can increase the readiness of generating unit production (EAF) by 3.33% in one MO event due to the replacement of the oil cooler generator transformer and lower maintenance costs for the required transformer oil cooler. The modified Oil Cooler was installed during the Maintenance Outage in Unit 1 and Overhaul unit 2 in 2021 and is still operating well for 20 months of operation.

CFD simulation of CO2 methanation through the Sabatier reaction in a shell-and-tube reactor incorporating phase change on the shell side

This paper reports a computational fluid dynamics simulation of carbon dioxide (CO2) methanation via the Sabatier process in a shell-and-tube reactor, incorporating a water phase change simulation on the shell side using the volume of fluid (VOF) method. The essential heat removal factors and their mechanisms, such as the catalyst size, bed porosity, thermal conductivity, coolant flow rate, and coolant boiling, were primarily investigated. The computational code was a newly developed in-house version of ChtMultiRegionFOAM based on OpenFOAM v2006. This code can simulate catalyst-gas heat transfer, which is called the two-temperature model. The code was extended to new methanation and phase-change models based on the VOF method. The computational code was first validated using existing experimental data. After sufficient accuracy was confirmed, several parametric studies were conducted. Catalyst heat conductivity was the most influential factor among the catalyst-related parameters. The flow rate of the oil coolant significantly affected the cooling performance. Finally, the use of boiling water significantly reduced the coolant flow rate. Moreover, the cooling performance remained unchanged until the total heat release rate from the gas exceeded the total latent heat of the water.

Role of CO2 cooling strategies in managing tool wear during the shoulder milling of grade 2 commercially pure titanium

As a result of their high hardness, osseointegration and low chemical reactivity titanium alloys have become an invaluable material species for use in medical applications. Unfortunately, many of the material properties which make titanium alloys desirable biomedical materials, also contribute towards difficulties in tool wear and surface quality during machining. Historically, this has been met via the application of robust metalworking fluid (MWF) strategies, e.g. flood, or high-pressure (HP) soluble oil coolant (emulsion). However, this brings with it its own challenges, not least of which the problem of coolant residue contamination. This study describes efforts to minimise and characterise tool wear during the shoulder milling of grade 2 commercially pure titanium (CP–Ti) by using a novel external supercritical CO2 (scCO2) coolant strategy. Whilst the results imply that the sole use of scCO2 coolant contributes to increased tool wear (relative to emulsion), an optimised strategy of combining scCO2 with minimum quantity lubrication (MQL) is shown to support up to a 245% increase in tool life. Despite this, machinability outcomes are found to be highly contingent upon proper selection of trial process variables such as scCO2 mass flow rate and cutting speed, wherein cutting speeds at or below 115 m/min, and 35 kg/h of scCO2 is generally shown to be the most efficacious parameters for this material/operation. Ultimately, scCO2 + MQL is found to be an effective compromise between the cleanliness of scCO2 and the improved function of emulsion.

Dynamic Heat Dissipation Model of Distributed Parameters for Oil-Directed and Air-Forced Traction Transformers and Its Experimental Validation

A traction transformer with narrow oil channels is usually cooled with the ODAF or "Oil Directed Air Forced" method, where its temperature greatly depends on the Joule heat of windings, the conjugate heat transfer in the transformer, and the secondary heat release via oil cooler, together with the oil flowrate generated by oil pump. Neither the thermal-electric analogy nor the CFD simulation approach is qualified to predict the temporal and spatial temperature variations in this type of transformer. In the current work, the distributed parameter models are built for traction transformers and oil coolers with the assumption of a one-dimensional temperature field in the oil flow direction, respectively. Then, the two models are combined with the lumped parameter ones of oil pumps and pipes via the flow rate, temperature and pressure continuities at their interfaces, resulting in the derivation of the dynamic heat dissipation model of oil-directed and air-forced traction transformers. Additionally, an efficient algorithm is proposed for its numerical solution, and the temperature rise experiment is performed for model validation. Finally, the fundamental of dynamic heat dissipation in traction transformers is investigated with the current numerical model and the effects of ambient temperature are studied.

Aerothermal characterization of finned surfaces in high-speed flows

Surface Air Cooled Oil Coolers are finned heat exchangers installed in the bypass duct of a turbofan engine that act as an additional cooling source for the engine components, having transonic flow as a heat sink. Characterizing the heat transfer and the aerodynamics of finned surface designs is essential to understanding their impact on overall engine efficiency. In the present study, different models are assessed numerically and experimentally. Two geometries are tested in a high-speed linear wind tunnel where measurements are taken using several sensors and optical techniques. An Inverse Heat Conduction Methodology based on a Levenberg-Marquardt Algorithm is developed to retrieve the heat flux on the fin’s surfaces accurately. A numerical analysis is performed using Reynolds Averaged Navier-Stokes simulations, and the results are compared against the experimental findings. A good agreement is observed between the datasets supporting the accuracy of the numerical methodology. Finally, a multi-objective optimization algorithm is coupled with the solver to explore new geometries that maximize the heat transfer while minimizing the pressure drop across the studied domain. Over 400 profiles were generated and numerically analyzed, allowing for identifying the features that have a more decisive influence on the performance of the fins. Profiles that present improvements of up to 19% are selected and analyzed in further detail. A combined improvement of 7.5% heat transfer enhancement and 9.1% reduction in pressure losses is achieved while maintaining a constant mass flow.

Thermal performance of the seal oil cooler of the RFCC MCB Pump by using CNT-water nanofluids

In this article, the water-based Carbon nanotube (CNT) nanofluid in various CNT concentrations was used in an industrial shell and tube cooler as a cooling fluid. The heat transfer factors have been analyzed, such as Reynolds number, Nusselt number, overall heat transfer coefficient, and heat transfer rate. The cooling fluid flows on the tube side, and the hot fluid flows on the shell side. The CNT concentrations were 0.0055%, 0.055%, 0.111% and 0.278% volume fraction. The results represented that the heat transfer properties increased with CNT volume fraction as 0.278 CNT volume fraction enhanced the Nusselt number and the convection heat transfer coefficient about 47% and 59.3%, respectively, for example. Likewise, heat transfer was enhanced with the increment of the nanofluid mass flow rate for all the CNT volume concentrations.

Analysis of the influence of temperature and hold time in the solid carburization process on the hardness and microstructure of AISI 1020 and 1045 using Oil Cooling

This study aims to determine the effect of variations in temperature and holding time on the value of hardness and microstructure in the carburizing process of low-carbon steel and medium-carbon steel. The temperature variations used during the carburizing process were 850oC and 950oC with variations of the holding time used being 15 minutes and 30 minutes. The types of materials used are AISI 1020 steel and AISI 1045 steel. In the carburizing process, the carbon source used is wood charcoal powder. After the process of adding carbon elements, the material will be quenched in the oil medium. The tests carried out were hardness testing and microstructure observations. The hardness test used the B-scale Rockwell hardness test method. The surface etching process of the material used a nital etching solution, namely a mixture of 3% nitric acid (HNO3) and 97% ethanol. The results showed that the lowest increase in hardness occurred at 850oC with a holding time of 15 minutes, namely 82.00 HRB for AISI 1020 steel and 95.66 HRB for AISI 1045 steel. Meanwhile, the highest increase in hardness occurred at a temperature of 950oC with a holding time of 30 minutes. namely 93.00 HRB on AISI 1020 steel and 105.33 HRB on AISI 1045 steel. This shows that the higher the temperature and the longer the holding time, the higher the hardness value.

EFFECTS OF SHOT PENNING ON MECHANICAL PROPERTIES, MICROSTRUCTURAL EVOLUTION AND RESIDUAL STRESS OF SUBMERGED FRICTION STIR-WELDED AA6061-T6 ALLOY

The purpose of this work is to study the effect of shot peening on the mechanical characteristics of welded plates which were performed by Submerged Friction Stir Welding (SFSW) on AA6061-T6 alloy. SFSW was carried out under three cooling mediums like water, brine solution, and coolant oil at the rotational speed of 1200[Formula: see text]rpm and welding speed of 45[Formula: see text]mm/min, respectively. The welded samples from each cooling medium were subjected to a shot peening process. A tensile test was carried out on weld peened and welds unpeened joints. The hardness of the various zones of HAZ, TMAZ, and NZ was also investigated. Optical and scanning electron microscopy was used to examine the microstructures of peened and unpeened joints formed by coolant samples. Shot peening has the potential to enhance the compressive residual stress of friction stir welded joints. According to these findings, shot peening had a greater impact on the mechanical properties of welded samples than unpeened samples, including tensile strength, percentage of elongation, and hardness. The grain size was a little bit smaller than the FSW sample because the surface of the welded samples was subjected to more strain during shot peening.

Designing The Estimation of The Need for Spare Parts and Inventory Policy on The D32 CP8 Compressor Machine

Based on damage data owned by PT XYZ, the compressor engine that has a history of high damage is the D32 CP8 Compressor Engine. Critical components of the D32 CP 8 Compressor are determined using a risk matrix. The critical components selected from the D32 CP8 Compressor Engine are Screw Motor, Refrigerant Air Cooler, and Cylinder Bearing Oil Cooler. This study uses the Reliability Centered Spares (RCS), Min-Max Stock, and Reorder Point (ROP) methods. The data collection and processing results obtained the need for critical components in the next 1 year based on the MTTF critical component data. These calculations show that the value of the required spare parts for the Motor Screw, Refrigerant Air Conditioner, and Cylinder Bearing Oil Cooler in a year is 8 components. The minimum Stock of the Screw Motor is 3 components, the maximum stock is 8 components, ReOrder Point point is 4 components. The minimum stock of 4 component air coolers, maximum stock of 7 components, ReOrder Point when 3 components. The minimum stock of a Cylindrical Bearing Oil Cooler is 2 components, the maximum stock is 6 components, and the ReOrder Point is when there are 2 components.

Analysis of Tensile Strength on ST.37 Material with SMAW Welding Variations of SAE 10 Oil and Water Cooling

In welding work must pay attention to the suitability of the welding construction in order to achieve optimal results. For this reason, welding needs to pay attention to several important things including welding efficiency, energy savings, and of course low costs. The purpose of this study was to determine how the tensile strength of the ST 37 material welded by SMAW welding which was cooled with a variety of cooling media, oil and pure water. The research method used in this research is to use real experimental research methods (True Experimental Research). From the results of the study, it can be concluded that the highest Ultimate Stress (Tu) tensile strength value is when the material is cooled with oil media, where the highest value is 365.15 N/mm2, followed by water cooling media oil, where the highest value is of 347.75 N/mm2, and the lowest is without cooling media where the highest value is 343.35 N/mm2. This is due to the thermal cycle that affects the microstructure of the material, this is what causes the tensile strength of the material to increase when a cooling medium is applied to the ST 37 welding material.

Analysis of the safety of pumping units of pumping stations of machine water lifting in the function of reliability indicators

The article presents ways of analyzing the safety of pumping units of pumping stations of machine water lifting in the function of reliability indicators, taking into account the complexity of analyzing the safety of pumping stations of machine water lifting and presenting the pumping unit of a pumping station as a "system", as well as taking into account the need to attract information about their functional scheme, information about the reliability indicators of the elements included in these schemes, information about the adopted system of maintenance and repair, as well as the criteria for the failure of such systems, structural schemes for calculating the reliability of the pumping unit have been developed. The developed structural scheme for calculating the reliability of a pumping unit is presented, consisting of an electric motor, a pump, a power supply network, a control device, a converter, a machine exciter or a semiconductor excitation system, a winding, active steel, an excitation winding, a collector-brush apparatus, a bearing, guide bearings, an oil cooler, an air cooler of an electric motor, a working ring, guide bearings, system lubrication of pump bearings. Structural diagrams are also given for four variants of the design layout of pumping units: each pumping unit supplies water to its pipeline; several pumping units operate in one common pressure pipeline; the main and backup pumping units are connected in series; pumping units are connected to a common pressure pipeline in series-parallel. Based on these structural schemes, formulas for determining the probability of trouble-free operation of the pumping unit and pumping units, considering their structural layout, are compiled. The article assumes that the uptime of the synchronous motor of the pumping unit is distributed exponentially, and the law of the repair time distribution corresponds to the gamma distribution.