Firat, Ilker

This study numerically investigates the first- and second-law thermodynamic performance of a circular tube equipped with novel angled vane vortex generators (AVVGs). The effects of vane angle (α = 10°, 20°, and 30°) and pitch ratio (PR = 5, 10, and 15) on heat transfer, friction loss (f), entropy generation (S˙gen′), exergy efficiency (ηEx), Bejan number (Be), and thermal performance factor (TPF) were analyzed using ANSYS Fluent 2021 with the SST k–ω turbulence model. Simulations were performed for water under turbulent flow conditions within a Reynolds number (Re) range of 12,770–20,824. The results demonstrated that AVVGs significantly enhanced convective heat transfer by intensifying flow mixing and disrupting the thermal boundary layer. The maximum Nusselt number (Nu) of 161.06 was achieved at PR = 5, α = 30°, and Re = 20,824, whereas the highest heat transfer enhancement ratio reached 1.37 compared with the plain tube. Although friction losses increased with decreasing pitch ratio and increasing vane angle, entropy generation due to heat transfer decreased considerably. The minimum total entropy generation (S'gen, tot = 0.1979 W m⁻1 K⁻1) occurred at PR = 5, α = 30°, and Re = 20,824. The highest thermal performance factor of 1.045 was obtained at PR = 15, α = 20°, and Re = 14,378, indicating the optimum thermohydraulic balance. Additionally, empirical correlations with prediction errors less than 7.2% were established for the Nusselt number, friction factor, and TPF. The proposed AVVG design demonstrates strong potential for improving the thermal and thermodynamic performance of compact heat exchanger systems.

Sezer, Şükrü; Sezer, Cihan; Celen, Ali; Bacak, Aykut; Dalkılıç, Ahmet Selim

The estimation of heat transfer coefficients (HTC) and pressure drop (ΔP) in flow boiling processes is essential for the effective design and operation of refrigeration systems. In this study, the artificial neural network (ANN), locally weighted regression (LWR), and gradient boosted machine (GBM) methods are employed to predict the boiling heat transfer coefficient (HTC) and pressure drop (ΔP) in flow boiling of R134a. The study focuses on horizontally positioned both straight and microfin tubes. The ANN, LWR, and GBM methodologies are utilized to ascertain the parameters of boiling HTC and ΔP as outputs. These parameters are determined by considering the mass flux, saturation pressure, heat flux, vapor quality, Reynolds number, Lockhart–Martinelli parameter, Froud number, Weber number, and Bond number as inputs. The training dataset is partitioned into 5 sections for the purpose of hyperparameter tweaking for each model. Out of these sections, 4 parts, consisting of approximately 111 samples, are utilized for training, while 1 part, including around 27 samples, is allocated for validation. The optimal hyperparameters are determined by calculating the average R2 score over the 5 validation sets. Using raw measurements, HTC and ΔP are successfully modeled using a relatively much smaller dataset of 174 measurements, with 82.4% R2 score and 0.7% weighted average relative deviation for HTC, and 88.9% R2 score and 4.1% weighted average relative deviation for ΔP across multiple tube types, achieved by LWR algorithm. Model performances are validated with an extrapolation test and found to be consistent with traditional train–validation–test sampling scheme with 75.9% R2 score and −6.2% weighted average relative deviation for HTC, and 89.3% R2 score and −3.9% weighted average relative deviation for ΔP, showing the consistency of the hypotheses created by a hybrid of parametric and nonparametric model families even outside the observed measurement range for multiple tube types. Local weighted regression models are the most performant, especially for limited data availability. However, calculated measurements increase error rates, suggesting that HTC and ΔP models work best with raw measurements.

Firat, Ilker; Karagoz, Sendogan; Yildirim, Orhan; Yilmaz, Mehmet

In this study, the effects on thermal performance and entropy production of new type turbulators with different blade angles and numbers, which are placed in a circular shaped pipe and manufactured in a 3D printer, were investigated experimentally. According to the experiments performed in the Reynolds number (Re) range of 7007–13,982, the Nusselt numbers of the 60°, 70° and 80° fin angle turbulators are, respectively, compared to the supported plain pipe; increased by 82.09%, 98% and 105.83%. Due to the increase in the blade angle, the maximum thermal performance was obtained as 1.542 for the 80° blade angle turbulator for Re = 12,538. The thermal performance factors of the 80° and 70° blade angle turbulators were found to be 1.048 and 1.039 times higher than the 60° blade angle turbulators, respectively. It was determined that the thermal improvement factor of the 80° blade angle turbulator is 1.008 times higher than the 70° blade angle turbulator. In terms of the second law, the maximum entropy production increase of the finned turbulators compared to the supported plain pipe was 153.96% at Re = 8387, in 3 turbulators with 80° fin angles. It has been determined that 1 turbulator model with 60° blade angles is more advantageous compared to the turbulators with 70° and 80° blade angles, since it produces the lowest entropy. In addition, it has been concluded that the most suitable turbulator model in terms of thermal performance is 3 turbulators with 80° blade angles. Graphic abstract: [Figure not available: see fulltext.]

Demir, Fatih; Turgut, Oguz; Calisir, Tamer

Flow field and heat transfer of an impinging swirling jet at low nozzle-to-plate distances have been investigated numerically for three different cases with six different turbulence models. The effects of Reynolds number (2100, 4100, 6100, 8100) and dimensionless nozzle-to-plate distance (H/D = 0.25, 0.5, 0.75, 1) on flow field and heat transfer of the swirling jet are studied parametrically. It is noted that the results of the cases employed exhibit sensitivity to the height of the computational domain defined on the impingement plate, particularly at low nozzle-to-plate distances. It is also seen that one of the cases used is in good agreement with the experimental results by employing Realizable k–ε turbulence model. Parametric analysis results show that the theoretical swirl number decreases with increasing Reynolds number at constant H/D and raises for H/D < 0.75. With the decrease in the Reynolds number from 8100 to 2100, although the H/D loses gradually effect on the heat transfer, H/D = 0.25 continues its effect. It is observed that the pressure peaks and the subatmospheric pressure on the impingement plate change with the nozzle-to-plate distance and Reynolds number.

Tabak, Ahmet; Sevimli, Kadir; Kaya, Mehmet; Çağlar, Bülent

The novel activated carbon materials were synthesized by the carbonization and chemical activation of agricultural bio-waste from tea woody stems (TWS), and these materials were characterized by using FTIR, SEM, TG/DTA and surface analysis (BET surface area; pore distribution; pore size, area and volume) techniques. The higher carbonization times and amount of chemical reagent generally produce the higher surface area at 400 and 500 °C. The experimental data demonstrated that the highest BET surface area was 789 m2 g−1 at 500 °C, 75 μm particle size, TWS/KOH ratio of 1:2 and carbonization time of 3 h. The activated carbon IR spectrum showed the increases in intense of aromatic C–H out-of-plane bending and C=C vibrations and the disappearance of olefinic C=C functional groups. Thermal analysis curves of TWS are ascribed to decomposition of organic derivate corresponding to lignin, cellulose and hemicelluloses components up to 700 °C from 200 °C. The SEM images and textural analysis data signified that the pore development of carbon skeleton structure during carbonization and activation process formed and type of micropores in these pore structures was dominant.