Study of Hot-air Recirculation around Off-road Tier-4 Diesel Engine Unit Using CFD Siddharth Jain, Yogesh Deshpande, Atul Bokane, Arun Kumar Santharam, and Deepak Babar, Halliburton Abbreviations: Computational fluid dynamics (CFD), boundary condition (BC). Keywords: Hot-air recirculation, Tier-4 diesel engine, recirculation zone, aerodynamic deflectors Abstract The goal of this study was to increase the cooling efficiency of Tier 4 diesel engine installations on well service equipment. These efficiency improvements were directed at reducing hot-air recirculation around the engine s radiators. Well service equipment units are designed for use in upstream oilfield activities. These units are powered by diesel engines that are required to comply with the new emission standards defined by the United States Environmental Protection Agency (EPA) for offroad engines. The new Tier 4 emission standards have led engine manufacturers to increase the cooling requirements for engine packages, thus increasing the burden on existing cooling systems. A traditional radiator uses ambient air and is influenced by parameters such as air temperature, wind direction, wind speed, and nearby objects. This equipment is intended to work in operating temperatures greater than 115 F but are currently limited in ambient capability. Particularly during the summer, these engines are prone to overheating. One cause of radiator inefficiency is the recirculation of hot air around the radiator, which significantly reduces cooling system performance. Computational fluid dynamics (CFD) modeling provides an effective means to assess the cause of recirculation and offers a way to evaluate solutions to improve unit performance. CFD modeling was performed, and based on the results, recirculation zones were identified. For a particular recirculation zone, several combinations of aerodynamic deflectors were designed to deflect hot air and prevent recirculation. Analysis was conducted using ANSYS FLUENT V13 software. The geometry creation of the computational domain was performed using SolidWorks 2011 software. The computational domain includes the radiator, fan, front cab, fuel tank, and simplified engine to study the effect of hot-air recirculation. Considering the effect of the radiator and fan, CFD analysis provides results in the form of velocity vectors and path lines, which provide actual flow characteristics of air circulation around the radiator. The CFD results were in excellent agreement with the data measured during physical testing. The CFD results indicated that adding deflectors would greatly reduce recirculation and improve unit performance, enabling the units to operate in higher ambient conditions. The output of this analysis is intended to be used at field locations across the globe for reliability in operation. Introduction and Background Recirculation has been an important concern during the last few decades in the oil and gas industry. General rules for predicting the occurrence of hot-air recirculation have been around for a considerable time [1, 2]. Hot-air recirculation occurs where the use of a heat transfer device is required, such as at power plants or in the oil and gas industry [1]. The most universal equipment used for cooling purposes in the oilfield service industry is radiators. Radiators are effective heat transfer devices, but, because they use ambient air, their performance is influenced by air temperature, wind direction, wind speed, and by the proximity of other air coolers and environmental flows. In particular, radiators can experience recirculation of hot exhaust air back to the air intake side,
significantly reducing the overall unit performance. With the expressive computer capability and extensive development in the simulation field, CFD has drawn attention in recent years. CFD models, if created correctly, can account for the complex interactions between the ambient conditions (wind speed and air temperature) and equipment and heated plumes that are often the cause of recirculation problems. CFD models have been used to evaluate recirculation zones and also to design deflectors to improve overall cooling performance. The models provided sufficiently accurate predictions over a range of operating conditions, which were not possible using other methods. With recent advances in computational speed and modeling capabilities, the complex three-dimensional geometries of the equipment can now be modeled with only minor simplifications. Problem Definition Every off-road diesel unit must comply with new emission standards as defined by the United States EPA. According to the EPA, new equipment built after Jan 1, 2011 must incorporate diesel engines that comply with the new Tier 4 standard. Specific diesel Tier-4 units are designed for use in upstream oilfield activities (Fig. 1). The unit discussed was intended to operate in the southern region of the U.S. where temperatures can reach 115 F regularly. Because this unit experienced air recirculation problems it was only capable of operating at temperatures below 104 F without engine overheating. This was the result of radiator inefficiency caused by recirculation of hot air around the Tier-4 unit and, thus, a significant reduction of cooling system performance. Fig. 1: Tier-4 Unit. Methodology CFD modeling provides an efficient way to evaluate the cause of recirculation and offers an approach to estimate keys to enhance unit performance. For this study, CFD analysis has been divided into three stages. 1. CFD of the fan. 2. CFD of the total computational domain. 3. Design optimization for the deflector. Figs. 2 and 3 show the computational domain for the fan and complete Tier-4 unit, respectively. Initially, the fan performance was evaluated by validating its speed and direction of rotation. The complete domain was considered to gain comprehensive information about recirculation patterns around the engine unit.
Turbulence Model K-Єturbulence model with standard wall function Temperature 298 K Pressure Atmospheric pressure Flow media Air 3D, Pressure Based Implicit, steady Fig. 2: Computational Domain Fan. Solution Method SIMPLE, First Order Upwind Convergence Criteria 1e -4 Meshing Extracting volume method with tetrahedral elements. Fig. 4: Simulation Parameter. Fig. 3: Computational Domain Complete Unit. CFD Setup Fig. 4 shows a typical simulation parameter used in ANSYS-FLUENT13.0. For modeling the fan, inlet pressure and outlet pressure were defined as boundary conditions (BCs). The speed (measured in rpm) of the fan was entered as an input. For the complete computational domain [fan + engine + radiator], BCs were set up for inlet velocity, inlet and outlet pressure, and the wall conditions. The input was provided in terms of velocity of air flow, pressure, fan rpm, and radiator resistance. Fig 5 shows meshing performed using an ANSYS workbench patch independent meshing option. It has 3.5 million tetrahedral elements to discretize computational domain. The quality of mesh was assured to be acceptable. Fig. 5: Meshing Computational Domain.
Deflector Locations and Design Profiles The deflector s location was decided after identifying the recirculation zone around the radiator. The deflectors were positioned In between the front cab and radiator On the bottom of the radiator On the side of the radiator (side deflector) Fig. 6 shows the locations for various deflectors, which were placed on the Tier-4 unit. Fig. 6: Deflectors Location on Tier-4 Unit. Results CFD analysis was performed for all three design profiles. The optimized profile of the deflectors was selected based on the hot-air recirculation zone and path line pattern. The straight and inclined deflector showed the hot air was again reversed back to the radiator inlet, and the recirculation zones formed at the bottom of the fuel tank. These results were not desirable to control hot-air recirculation. However, with the curved deflector, due to its aerodynamic nature, most of the air contacts the curved trajectory, which directs the hot air flow away from the radiator and engine inlet. This improved the unit performance significantly. It was decided to use the curved deflector to limit hot-air circulation problems around the Tier-4 engine. The length and size of the curved deflector was designed as per material availability and manufacturability. Figs. 8a, 8b, and 8c clearly show the recirculation of air around the Tier-4 unit without any deflector. The velocity vector showed the flow of hot air was forming recirculation zones at the radiator inlet and around the engine, which caused engine overheating. Eventually, this resulted in poor unit cooling performance. Fig. 7 shows the design profiles of the various deflectors. CFD analysis was performed for the following designs: Straight deflectors. Inclined deflectors. Curved deflectors Fig. 8: (a) Velocity Vectors Domain without Fig. 7: Deflector s Designs Profile.
Fig. 8: (b) Velocity Vectors Domain without Fig. 9: (a) Velocity Vectors Domain with Fig. 8: (c) Velocity Vectors Domain without On the basis of CFD analysis with the base domain without a deflector, several recirculation zones were identified. The curved deflector was designed in succession over straight and inclined deflectors. Fig. 9: (b) Velocity Vectors Domain with It was observed that, once deflectors were placed at their respective positions, the recirculation of the hot air was limited, thereby improving the reliability of the unit. Figs. 9a through 9d show the flow around the radiator, and it is apparent from the velocity vectors that hot air was deflected away from the radiator and engine unit by the deflector. Fig. 9: (c) Velocity Vectors Domain with
Fig. 9 (d) Velocity Vectors Domain with Conclusions The hot-air recirculation around the Tier- 4 unit was predominantly causing poor cooling performance and frequent potential engine overheating conditions. CFD was an effective and comprehensive tool to evaluate this phenomenon by considering a larger computational model of the entire Tier-4 unit and its surrounding components. The base computational model with the Tier-4 unit evaluated flow characteristics in terms of flow direction of hot air, path lines, and velocity vectors. The locations and different configurations of deflectors to limit the hot-air recirculation toward the radiator and engine inlet were identified. Various designs of deflectors, including straight, vertical, inclined, and curved, were studied to reduce the hot-air recirculation in the Tier-4 units. It was determined that the curved deflector was more effective and provided the best solution by restricting the hot-air recirculation. Recirculation of hot air occurring around a Tier-4 unit was evaluated using ANSYS FLUENT 13. ANSYS FLUENT 13 has excellent meshing capability for complex and large domain volume. CFD results were presented in the form of velocity vectors and path lines, which provide actual flow characteristics of air circulation around the radiator. Several combinations of deflectors were designed as straight, inclined, and curved deflectors to limit hot-air recirculation around a Tier-4 unit. Particularly, the curved deflectors showed significant improvement to control hot-air recirculation in comparison to straight, vertical, and inclined deflectors. The CFD results were in excellent agreement with the data measured during physical testing. Adding deflectors would reduce recirculation and improve performance, enabling the units to operate in severe conditions. Acknowledgement The authors thank the management of Halliburton for their support and permission to publish this paper. They also express gratitude to all of the team members who contributed to the job design, preparation, and execution of the operation to achieve the results presented in this paper. The key and important outcomes of this study are identified below.
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