بررسي اهميت سازو كار مهاجرت گرمايي در مبدل هاي حرارتي اسفنجي با استفاده از شبيه سازي

INVESTIGATION OF THERMOPHORESIS IMPACT ON FOAMED HEAT EXCHANGERS USING SIMULATION

مبدل هاي حرارتي با ساختار اسفنج فلزي مزيت هاي بالقوه فراوان دارند اما، مهم ترين چالش اين گونه مبدل ها تشكيل رسوب ذره يي بر اثر سازوكار مهاجرت گرمايي (ترموفورسيس) است. در اين مطالعه عملكرد ترموهيدروليكي مبدل حرارتي اسفنجي در حالت تميز، تحت شرايط جابه جايي اجباري، گراديان دمايي و سرعت هاي گوناگون، به منظور بررسي وجود يا عدم وجود سازوكار مهاجرت گرمايي، با كمك مدل سازي عددي شبيه سازي و با داده هاي آزمايشگاهي اعتبارسنجي شده است. نتايج شبيه سازي عددي نشان مي دهد كه اختلاف دماي فاز سيال و جامد در ضخامت هاي مختلف اسفنج ناچيز است. بر اين مبنا و با توجه به اين كه بيشينه نيروي محركه ي سازوكار مهاجرت گرمايي در حالت تميز مبدل وجود دارد، مي توان گفت كه به دليل تعادل گرمايي بين فاز سيال و جامد، اختلاف دما بين دو فاز ناچيز بوده و در نتيجه در مبدل داراي ساختار اسفنج آلومينيومي، سازوكار مهاجرت گرمايي نقشي در رسوب ريزذرات روي سطوح الياف تحت شرايط جابه جايي اجباري ندارد.

Concerns about emissions and desire for their reduction have resulted in changes in automotive engine combustion systems. New developed EGR systems require high temperature and high-performance compact heat exchangers as well as capability of operating in harsh environments. Foams offer a large surface area per unit volume as well as high material thermal conductivity which would promote fluid mixing, thereby improving the overall performance of the heat exchanger. More interestingly, it is demonstrated that the foams can be cleaned easily without relying on expensive cleaning techniques. These interesting characteristics, along with recent improvements in foam fabrication methods, have resulted in several research efforts on the use of foams in compact heat exchangers, especially new EGR systems. One of the principal challenges is the deposition of particulate matter mainly as a result of thermophoresis in non-isothermal systems. Accordingly, in the present study, to investigate the significance of thermophoresis in open cellular metal foam, Two-Dimensional (2D) numerical simulations of a channel partially filled with aluminium foam as an EGR cooler were performed by ANSYS FLUENT 16.0 to solve local thermal non-equilibrium equation under clean conditions (maximum driving force of particle transport for thermophoresis mechanism). The attempted foam is made of aluminium and is embedded to the channel, meaning that the conduit is only filled with foam partially to compensate high pressure due to blockage. Thermo-hydraulic performance for a foam with density of 20 PPI is examined under different velocities and thermal gradients. The numerical results are compared with those of experiments. Under non-isothermal conditions, the numerical results confirmed that temperature gradient would be marginal for different thicknesses of aluminium foams because thermal equilibrium has been established, especially at foam and foam-free interface. Provided that maximum driving force for thermophoresis occurs at clean conditions, it is expected that this mechanism would have minimal impact at fouling conditions where particulate matter passes through the channel.