Development of a new near-wall reynolds stress turbulence model for jet impingement heat transfer prediction

U ovom radu je predstavljen nov naponski model (model drugog reda) turbulentnih napona. Ovaj novi naponski model nastao je pretvaranjem "standardnog" IP (high-Reynolds) naponskog modela u odgovarajući naponski model kojim je moguće vršiti proračune i u oblastima strujanja sa malim vrednostima Rejnoldsovog turbulentnog broja (low-Reynolds model), kao i sa popravkom tog modela u vidu dopunskog člana nagle preraspodele turbulentnih napona usled prisustva zida *R II, w τ, ij. Pretvaranje IP modela iz njegove high-Reynolds u njegovu low-Reynolds verziju, izvršeno je uključivanjem prethodno zanemarenog uticaja molekularne difuzije na procese prenošenja, tj. sa uvođenjem odgovarajućih članova i funkcija u jednačinu "prenošenja" Rejnolsovih napona i jednačinu "prenošenja" disipacije turbulentne kinetičke energije. Novi, dopunski član nagle preraspodele turbulentnih napona usled prisustva zida *R II, w τ, ij, koji je modeliran u skladu sa realnom fizičkom situacijom, obuhvatio je netipičan takozvani efekat eha pritiska, tj. netipičan proces preraspodele turbulentnih napona koji se javlja u strujnom polju pri udaru mlaza o ploču u blizini zaustavne tačke. Nasuprot "standardnim" linearnim dvojednačinskim modelima turbulentnih napona, predloženi naponski model daje kvalitativno bolja predviđanja polja kinetičke energije turbulencije i značajno bolja predviđanja lokalnih vrednosti Nuselovog broja. U poređenju sa "standardnim" high-Reynolds naponskim modelima, predloženi model pokazuje značajno bolja predviđanja turbulentnih napona u zaustavnoj zoni udara mlaza, nešto bolja predviđanja polja osrednjenih brzina, a i omogućava predviđanje lokanih vrednosti Nuseltovog broja.The newly proposed Reynolds-stress turbulence model (second moment closure) was created by transforming the "standard" high-Reynolds Isotropisation-of-Production turbulence model into its low-Reynolds version and by introducing a new additional wall-reflection term, *R II, w τ, ij Transformation from high- to low-Reynolds turbulence model was carried out by including the previously neglected influence of molecular diffusion i.e. by introducing the appropriate terms and functions into Reynolds stress and turbulent dissipation rate transport equation. The new additional "rapid" wall-reflection term *R II, w τ, ij, that was modeled in accordance to the real physical situation, encompassed the "atypical" so-called pressure-echo effect, i.e. the "atypical" redistribution of turbulent stress in the vicinity of the stagnation point of an impinging jet. In contrast to "standard" linear near-wall two-equation turbulence models, the newly proposed Reynolds-stress turbulence model gives essentially better predictions of turbulent kinetic energy field and considerably better predictions of local Nusselt number. Compared with the "standard" high Reynolds turbulence stress models, the proposed turbulence model demonstrates considerably better prediction of turbulent stress field in the vicinity of impinging jet stagnation point, slightly better prediction of mean velocity field, and also enables prediction of local Nusselt number

Development of a new near-wall reynolds stress turbulence model for jet impingement heat transfer prediction

U ovom radu je predstavljen nov naponski model (model drugog reda) turbulentnih napona. Ovaj novi naponski model nastao je pretvaranjem "standardnog" IP (high-Reynolds) naponskog modela u odgovarajući naponski model kojim je moguće vršiti proračune i u oblastima strujanja sa malim vrednostima Rejnoldsovog turbulentnog broja (low-Reynolds model), kao i sa popravkom tog modela u vidu dopunskog člana nagle preraspodele turbulentnih napona usled prisustva zida *R II, w τ, ij. Pretvaranje IP modela iz njegove high-Reynolds u njegovu low-Reynolds verziju, izvršeno je uključivanjem prethodno zanemarenog uticaja molekularne difuzije na procese prenošenja, tj. sa uvođenjem odgovarajućih članova i funkcija u jednačinu "prenošenja" Rejnolsovih napona i jednačinu "prenošenja" disipacije turbulentne kinetičke energije. Novi, dopunski član nagle preraspodele turbulentnih napona usled prisustva zida *R II, w τ, ij, koji je modeliran u skladu sa realnom fizičkom situacijom, obuhvatio je netipičan takozvani efekat eha pritiska, tj. netipičan proces preraspodele turbulentnih napona koji se javlja u strujnom polju pri udaru mlaza o ploču u blizini zaustavne tačke. Nasuprot "standardnim" linearnim dvojednačinskim modelima turbulentnih napona, predloženi naponski model daje kvalitativno bolja predviđanja polja kinetičke energije turbulencije i značajno bolja predviđanja lokalnih vrednosti Nuselovog broja. U poređenju sa "standardnim" high-Reynolds naponskim modelima, predloženi model pokazuje značajno bolja predviđanja turbulentnih napona u zaustavnoj zoni udara mlaza, nešto bolja predviđanja polja osrednjenih brzina, a i omogućava predviđanje lokanih vrednosti Nuseltovog broja.The newly proposed Reynolds-stress turbulence model (second moment closure) was created by transforming the "standard" high-Reynolds Isotropisation-of-Production turbulence model into its low-Reynolds version and by introducing a new additional wall-reflection term, *R II, w τ, ij Transformation from high- to low-Reynolds turbulence model was carried out by including the previously neglected influence of molecular diffusion i.e. by introducing the appropriate terms and functions into Reynolds stress and turbulent dissipation rate transport equation. The new additional "rapid" wall-reflection term *R II, w τ, ij, that was modeled in accordance to the real physical situation, encompassed the "atypical" so-called pressure-echo effect, i.e. the "atypical" redistribution of turbulent stress in the vicinity of the stagnation point of an impinging jet. In contrast to "standard" linear near-wall two-equation turbulence models, the newly proposed Reynolds-stress turbulence model gives essentially better predictions of turbulent kinetic energy field and considerably better predictions of local Nusselt number. Compared with the "standard" high Reynolds turbulence stress models, the proposed turbulence model demonstrates considerably better prediction of turbulent stress field in the vicinity of impinging jet stagnation point, slightly better prediction of mean velocity field, and also enables prediction of local Nusselt number

Improvement of cfd models of tunnel fire development based on experimental data

This paper, dealing with the problems of mathematical description of the tunnel fire development process with the use of experimental data, outlines the procedure of correction of the existing and obtaining of an improved CFD model package. The improved CFD model was developed on the basis of detailed analysis and comparison of experimental and numerical results, through consideration of the physical structure of all processes affecting combustion. During the analysis it was noticed that the existing CFD model in the part covering combustion based on the so-called steady laminar flamelet model, treats the combustion process almost as a direct correlation between the processes of mixing gasses and heat release rate. This potential deficiency has been overcome by correction of the model in the section defining boundary condition for the burning surface and by establishing a direct correlation between the measured value of the fuel mass change rate and the amount of heat released from burning surface. In this way a modification of complex stoichiometric combustion processes was avoided, while providing the model that better describes and predicts the course of events in this type of complex, anisotropic and turbulent flow of gases in the tunnel

Absence of system xc⁻ on immune cells invading the central nervous system alleviates experimental autoimmune encephalitis

Background: Multiple sclerosis (MS) is an autoimmune demyelinating disease that affects the central nervous system (CNS), leading to neurodegeneration and chronic disability. Accumulating evidence points to a key role for neuroinflammation, oxidative stress, and excitotoxicity in this degenerative process. System x(c)- or the cystine/glutamate antiporter could tie these pathological mechanisms together: its activity is enhanced by reactive oxygen species and inflammatory stimuli, and its enhancement might lead to the release of toxic amounts of glutamate, thereby triggering excitotoxicity and neurodegeneration. Methods: Semi-quantitative Western blotting served to study protein expression of xCT, the specific subunit of system x(c)-, as well as of regulators of xCT transcription, in the normal appearing white matter (NAWM) of MS patients and in the CNS and spleen of mice exposed to experimental autoimmune encephalomyelitis (EAE), an accepted mouse model of MS. We next compared the clinical course of the EAE disease, the extent of demyelination, the infiltration of immune cells and microglial activation in xCT-knockout (xCT(-/-)) mice and irradiated mice reconstituted in xCT(-/-) bone marrow (BM), to their proper wild type (xCT(+/+)) controls. Results: xCT protein expression levels were upregulated in the NAWM of MS patients and in the brain, spinal cord, and spleen of EAE mice. The pathways involved in this upregulation in NAWM of MS patients remain unresolved. Compared to xCT(+/+) mice, xCT(-/-) mice were equally susceptible to EAE, whereas mice transplanted with xCT(-/-) BM, and as such only exhibiting loss of xCT in their immune cells, were less susceptible to EAE. In none of the above-described conditions, demyelination, microglial activation, or infiltration of immune cells were affected. Conclusions: Our findings demonstrate enhancement of xCT protein expression in MS pathology and suggest that system x(c)- on immune cells invading the CNS participates to EAE. Since a total loss of system x(c)- had no net beneficial effects, these results have important implications for targeting system x(c)- for treatment of MS

Friction and wear processes-thermodynamic approach

Tribology, as the scientific and professional discipline within the mechanical engineering, studies phenomena and processes on the interacting surfaces, in direct and indirect contact and in relative motion. It includes the study and application of the principles of friction, wear and lubrication, as well as phenomena connected with these processes. Given that a process involving friction is always accompanied by transformation of energy, more precisely an energy dissipation process which generates entropy, the concept of thermodynamic entropy production analysis represents one of appropriate tools for studying and analysing the behaviour of complex friction and wear processes. This paper presents a review of published works in which the thermodynamic approach was used in analysing the friction and wear processes in tribosystems

Low temperature hydronic heating system with radiators and geothermal ground source heat pump

Upotreba grejnog sistema s toplotnom pumpom i zemljom kao izvorom energije bez obzira na relativno nisku temperaturu raspoloživog "izvora" geotermalne energije (temperatura vode ili tla ne mora biti viša od 12°C), omogućava da se 50-80% energije potrebne za grejanje preuzme od zemlje, a da se ostatak nadomeštava električnom energijom. Ovaj udeo geotermalne energije, u ukupnoj energiji potrebnoj za grejanje, zavisi pre svega od sistema njene dalje raspodele (radijatorski sistem, sistem grejanja pomoću fancoil-a ili zidno i podno grejanje), a tek potom i od temperature vode, odnosno tla. U radu je data uporedna analiza termičke efikasnosti dva sistema centarlnog grejanja sa radijatorima kao grejnim telima, visokotemperturnog kotlovskog sistema grejanja (90°C/70°C) i niskotemperaturnog sistema sa toplotnom pumpom i zemljom kao izvorom energije (65°C/55°C, 55°C/45°C ili 50°C/45°C).The use of a system with a geothermal ground source heat pump, regardless of the relatively low temperature of the available source of geothermal energy (water or ground temperature does not need to exceed 12°C), allows 50-80% of the energy required for heating to be taken from the ground, while the remaining amount is provided by electrical energy. This share of geothermal energy, in the total energy required for heating, primarily depends on the system of its further distribution (radiator system, fan coil heating system or wall and floor heating), and secondarily on water and ground temperature. The paper deals with the comparative analysis of thermal efficiency of two water central-heating systems with radiators, a conventional high-temperature heating system with a boiler (90°C/70°C) and a low-temperature heating system with a geothermal ground source heat pump ( 65°C/55°C, 55°C/45°C or 50°C/45°C)

Analytical and experimental determination of the temperature field on the surface of wall heating panels

This paper presents experimental verification of the accuracy and acceptability of Faxen-Rydberg-Huber analytical expression for determining thermal characteristics of the heating panels. The verification of accuracy of the Faxen-Rydberg-Huber expression, which describes the 2-D temperature field in the wall with series of embedded heated pipes having uniform temperature, was performed by comparing the temperatures on the surface of three types of heating wall panels, differing in structure and geometric characteristics. The analysis of results has shown high accuracy of Faxen-Rydberg-Huber expression in describing the temperature field. Also, it was noted that small changes in heating fluid temperature, occurring along the pipe in the heated panel, have a negligible influence on the accuracy of prediction by the expression. This confirmed that the Faxen-Rydberg-Huber expression can be used to describe the temperature field in the wall heating panels. At the same time, this expression has proven to be extremely sensitive primarily to thermophysical characteristics of the panel layers, as well as to the geometric parameters of the panels

Low temperature hydronic heating system with radiators and geothermal ground source heat pump

Upotreba grejnog sistema s toplotnom pumpom i zemljom kao izvorom energije bez obzira na relativno nisku temperaturu raspoloživog "izvora" geotermalne energije (temperatura vode ili tla ne mora biti viša od 12°C), omogućava da se 50-80% energije potrebne za grejanje preuzme od zemlje, a da se ostatak nadomeštava električnom energijom. Ovaj udeo geotermalne energije, u ukupnoj energiji potrebnoj za grejanje, zavisi pre svega od sistema njene dalje raspodele (radijatorski sistem, sistem grejanja pomoću fancoil-a ili zidno i podno grejanje), a tek potom i od temperature vode, odnosno tla. U radu je data uporedna analiza termičke efikasnosti dva sistema centarlnog grejanja sa radijatorima kao grejnim telima, visokotemperturnog kotlovskog sistema grejanja (90°C/70°C) i niskotemperaturnog sistema sa toplotnom pumpom i zemljom kao izvorom energije (65°C/55°C, 55°C/45°C ili 50°C/45°C).The use of a system with a geothermal ground source heat pump, regardless of the relatively low temperature of the available source of geothermal energy (water or ground temperature does not need to exceed 12°C), allows 50-80% of the energy required for heating to be taken from the ground, while the remaining amount is provided by electrical energy. This share of geothermal energy, in the total energy required for heating, primarily depends on the system of its further distribution (radiator system, fan coil heating system or wall and floor heating), and secondarily on water and ground temperature. The paper deals with the comparative analysis of thermal efficiency of two water central-heating systems with radiators, a conventional high-temperature heating system with a boiler (90°C/70°C) and a low-temperature heating system with a geothermal ground source heat pump ( 65°C/55°C, 55°C/45°C or 50°C/45°C)

Analytical and experimental determination of the temperature field on the surface of wall heating panels

This paper presents experimental verification of the accuracy and acceptability of Faxen-Rydberg-Huber analytical expression for determining thermal characteristics of the heating panels. The verification of accuracy of the Faxen-Rydberg-Huber expression, which describes the 2-D temperature field in the wall with series of embedded heated pipes having uniform temperature, was performed by comparing the temperatures on the surface of three types of heating wall panels, differing in structure and geometric characteristics. The analysis of results has shown high accuracy of Faxen-Rydberg-Huber expression in describing the temperature field. Also, it was noted that small changes in heating fluid temperature, occurring along the pipe in the heated panel, have a negligible influence on the accuracy of prediction by the expression. This confirmed that the Faxen-Rydberg-Huber expression can be used to describe the temperature field in the wall heating panels. At the same time, this expression has proven to be extremely sensitive primarily to thermophysical characteristics of the panel layers, as well as to the geometric parameters of the panels

Size distribution of agglomerates of milk powder in wet granulation process in a vibro-fluidized bed

Results of experiments on the influence of technological parameters (intensity of vibration, granulation of the liquid feed, temperature of fluidization agent) on the change of size distribution, as well as mass mean diameter of the milk powder particles subjected to the wet granulation process (agglomeration) in a vibro-fluidized bed granulator are shown in this paper. Using water as a granulation liquid and air as a fluidization agent, it was found that mass mean diameter increases with increase of water feed, intensity of vibration, and decrease of air temperature. Increasing the intensity of vibration and decreasing the air temperature, primarily induces the increase of the dimensions of the initial nuclei. This can be explained on the basis of different influences that these changes (velocity of particle motion, intensity of particle collision, drying rate) have on the coalescence of particles with smaller and/or bigger dimensions