Determination of heat transfer coefficients on the surfaces of plastic tubes

2021

Energies

Airflow occurring in a ventilation duct is characterized by low velocity and hence low Reynolds number. In these conditions, either a laminar, transitional or turbulent flow will occur. Different flow conditions result in different values of the friction coefficient. To achieve the transitional flow in numerical simulation, a modified algebraic model for bypass transition (modified k−ω) was used. Numerical simulation was validated using Particle Tracking Velocimetry (PTV) in the circular channel. The modified algebraic model consists of only two partial differential equations, which leads to much faster calculation than the shear stress transport model. Results of the modified algebraic model are largely consistent with either the measurement and shear stress transport model considering laminar and transitional flow. Consistency slightly decreased in turbulent flow in relation to the model using shear stress transport method.

Section: Introductionmentioning

confidence: 99%

Validation of Modified Algebraic Model during Transitional Flow in HVAC Duct

2021

Energies

“…It is known from the literature that the friction factor, coefficients of heat and mass transfer take in the areas of laminar-turbulent transition have much higher values than the corresponding coefficients characteristic for laminar or turbulent flow (Gnieliński, 1999; Kandlikar et al , 2014; Rup et al , 2002). In Rup et al (2002), the determined turbulent exchange of momentum and heat during flows in the inlet area of plastic pipes was analysed. Due to the small diameters of pipes, the flows analysed were described by small values of the Reynolds number 5,000 < Re < 50,000.…”

Section: Introductionmentioning

confidence: 99%

Modified algebraic model of laminar-turbulent transition for internal flows

Rup

2019

HFF

Purpose For internal flows with small values of the Reynolds number, there is often at a considerable distance from the pipe inlet cross-section a change of the flow form from laminar to turbulent. To describe this phenomenon of laminar-turbulent transition in the pipe, also parallel-plate channel flow, a modified algebraic intermittency model was used. The original model for bypass transition developed by S. Kubacki and E. Dick was designed for simulating bypass transition in turbomachinery. Design/methodology/approach A modification of mentioned model was proposed. Modified model is suitable for simulating internal flows in pipes and parallel-plate channels. Implementation of the modified model was made using the OpenFOAM framework. Values of several constants of the original model were modified. Findings For selected Reynolds numbers and turbulence intensities (Tu), localization of laminar breakdown and fully turbulent flow was presented. Results obtained in this work were compared with corresponding experimental results available in the literature. It is particularly worth noting that asymptotic values of wall shear stress in flow channels and asymptotic values of axis velocity obtained during simulations are similar to related experimental and theoretical results. Originality/value The modified model allows precision numerical simulation in the area of transitional flow between laminar, intermittent and turbulent flows in pipes and parallel-plate channels. Proposed modified algebraic intermittency model presented in this work is described by a set of two additional partial differential equations corresponding with k-omega turbulence model presented by Wilcox (Wilcox, 2006).

“…The reasons for this inconsistency are most likely changes in the velocity profile and significant amplitude of disturbances in the inlet cross-section. The laminar-turbulent transition region is distinguished by higher values of friction factor and heat transfer coefficient compared to those in the region of fully developed laminar or turbulent flow [3][4][5][6]. Rup et al in [3] analyses the statistical steady turbulent exchange of momentum and heat in the range of small values of Reynolds number (5000 < Re < 50000) implemented in the inlet region of a plastic pipe.…”

Section: Introductionmentioning

confidence: 99%

“…The laminar-turbulent transition region is distinguished by higher values of friction factor and heat transfer coefficient compared to those in the region of fully developed laminar or turbulent flow [3][4][5][6]. Rup et al in [3] analyses the statistical steady turbulent exchange of momentum and heat in the range of small values of Reynolds number (5000 < Re < 50000) implemented in the inlet region of a plastic pipe. Research described in [3] includes changes in turbulence intensity in the inlet cross-section and changes in the turbulent Prandtl number, Pr t , in the flow area.…”

Section: Introductionmentioning

confidence: 99%

“…Rup et al in [3] analyses the statistical steady turbulent exchange of momentum and heat in the range of small values of Reynolds number (5000 < Re < 50000) implemented in the inlet region of a plastic pipe. Research described in [3] includes changes in turbulence intensity in the inlet cross-section and changes in the turbulent Prandtl number, Pr t , in the flow area.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

An improved algebraic model for by-pass transition for calculation of transitional flow in pipe and parallel-plate channels

Rup

2019

THERM SCI

Original scientific paper https://doi.org/10.2298/TSCI19S4123N Modified algebraic intermittency model developed by E. Dick and S. Kubacki was used to describe laminar-turbulent transition. In this work a modification of this model was made for simulating internal flows in pipes and parallel-plate channel. In particular, constants present in this model were modified. These modified constants are the same for different flows in pipes and parallel-plate channels. In this work, a dependence of friction factor on Reynolds number and turbulence intensity were determined as well as the localization of laminar breakdown and fully developed flow. Obtained results were compared with theoretical and experimental data presented in the literature.