Nu Max Research Articles

Convection Heat Transfer and Performance Analysis of a Triply Periodic Minimal Surface (TPMS) for a Novel Heat Exchanger

Heat exchangers are necessary in most engineering systems that move thermal energy from a hot source to a colder location. The development of additive manufacturing technology facilitates the design and optimization of heat exchangers by introducing triply periodic minimal surface (TPMS) structures. TPMSs have shown excellent mechanical and thermal performance, which can improve heat energy transfer efficiency in heat exchangers. This current study intends to design and develop efficient, lightweight heat exchangers for aerospace and space applications. Using the TPMS structure, a porous construction encloses a horizontal tube that circulates heated fluid. Low-temperature water circulates inside a rectangular box that houses the complete system to remove heat from the horizontal pipe. Three porous structures, the gyroid, diamond, and FKS structures, were employed and examined. Porous models with various porosities and surface areas (15 cm2 and 24 cm2) were investigated. The results revealed that the gyroid structure exhibits the highest Nusselt number for heat removal (Nu max = 2250), confirming the highest heat transfer and lowest pressure drop among the three structures under investigation. The maximum Nusselt number obtained for the FKS structure is less than 1000, whereas, for the diamond structure, it is near 1250. A linear variation in the average Nusselt number as a function of the structure surface area was found for the FKS and diamond structures. In contrast, nonlinearity was observed in the gyroid structures.

Thermo-hydraulic performance of wavy microchannel heat sink with oblique grooved finned

• The combination of wavy and oblique grooves finned was analyzed numerically. • Wetted area governs the appropriate pattern type. • Nu max occurs in case of P = 375 and l = 250 µm. • Maximum pressure drop is created in case of P = 375 and l = 250 µm. • The secondary flow is one of the main contributors to heat transfer enhancement. • The performance index determined the effective pattern. One of the crucial phenomena to enhance the heat transfer of microchannel heat sinks is the flow mixing and wall interaction through the secondary flow. In the present study, a combination of the wavy and the oblique grooved microchannel patterns was investigated to improve the performance of the microchannel heat sink. The oblique grooves with a pitch of 375 µm were combined with the wavy microchannel. The effect of fins with widths of 250 and 125 µm on thermal performance was investigated (Type-1 and Type-2). The amplitude of 250 µm and wavelength of 2500 µm were considered for wavy microchannels. Furthermore, the effect of pitches on the heat transfer was studied; subsequently, pitches of 750 and 1500 µm were analyzed (Type-3 and Type-4). Nusselt number, pressure drop, and performance evaluation criteria index were reported as the results. The results showed that secondary flow generation causes the thermal boundary layer to re-develop at each fin, continually developing fluid flow. Type-2 had the maximum average Nusselt number of approximately 80 and a pressure drop of approximately 50 kPa at Re = 850 compared to the other three cases. The performance evaluation criterion index (ƞ) was calculated for all cases to achieve a comprehensive conclusion about the performance of a heat sink. The results showed that in Type-2, the heat transfer increase exceeded the pressure drop penalty, and its eta was almost 2.5. Fluid flow patterns were investigated in detail as the most significant reason for efficiency improvement, and the wetted area was studied, which their maximum value equaled 1.2 at Type-2.

THE RELATIONSHIP BETWEEN νmaxAND AGEtFROM ZAMS TO RGB-TIP FOR LOW-MASS STARS

Stellar age is an important quantity in astrophysics, which is useful for many fields both in the universe and galaxies. It cannot be determined by direct measurements, but can only be estimated or inferred. We attempt to find a useful indicator of stellar age, which is accurate from the zero-age main sequence to the tip of red giant branch for low-mass stars. Using the Yale Rotation and Evolution Code (YREC), a grid of stellar models has been constructed. Meanwhile, the frequency of maximum oscillations' power nu(max) and the large frequency separation Delta nu are calculated using the scaling relations. For the stars, the masses of which are from 0.8M(circle dot) to 2.8M(circle dot), we can obtain the nu(max) and stellar age by combing the scaling relations with the four sets of grid models (YREC, Dotter et al., Marigo et al., and YY isochrones). We find that nu(max) is tightly correlated and decreases monotonically with the age of the star from the main sequence to the red giant evolutionary stages. Moreover, we find that the line shapes of the curves in the Age versus nu(max) diagram, which is plotted by the four sets of grid models, are consistent for red giants with masses from 1.1 M-circle dot to 2.8 M-circle dot. For red giants, the differences of correlation coefficients between Age and nu(max) for different grid models are minor and can be ignored. Interestingly, we find two peaks that correspond to the subgiants and bump of red giants in the Age versus nu(max) diagram. By general linear least-squares, we make the polynomial fitting and deduce the relationship between log(Age) and log(nu(max)) in red giants' evolutionary state.

Average and extremal properties of heat transfer and shear stress on a wall surface in Rayleigh–Bénard convection

In this study surface-averaged and extremal properties of heat transfer and shear stress on the upper wall surface of Rayleigh–Benard convection are numerically examined. The Prandtl number was raised up to 103, and the Rayleigh number was changed between 104 and 107. As a result, average Nusselt number Nu and shear rate τ/Pr depends on Pr, Ra, and the entire numerical results are distributed between two correlation equations corresponding to small and large Pr. The small and large Pr equations are closely related to steady and unsteady flow regimes, respectively. Nevertheless, a single relation τ/Pr ~ Nu 3.0 exists to explain the entire results. Similarly the change of local maximal properties Nu max and τ max/Pr depends on Pr, Ra, and these values are also distributed between two correlation equations corresponding to small and large Pr cases. Despite such complicated dependence we can obtain a correlation equation as a form of τ max/Pr ~ Nu max 2.6 , which has not been obtained theoretically.

How Many Conformers Determine the Thymidine Low-Temperature Matrix Infrared Spectrum? DFT and MP2 Quantum Chemical Study

A comprehensive conformational analysis of isolated 2'-beta-deoxy-thymidine (T), canonical DNA nucleoside, has been performed by means of ab initio calculations at the MP2/6-311++G(d,p)//DFT B3LYP/6-31G(d,p) level of theory. At 298.15 K, all 92 conformers of isolated dT are within a 7.49 kcal/mol Gibbs energy range. Syn orientation for the base and South (S) conformers for the sugar dominate at this temperature: syn/anti = 61.6%:38.4% and S/N = 74.5%:25.5%. However, at 420 K, the majority of conformers contain anti base and the population of North (N) sugars increases: syn/anti = 38.0%:62.0% and S/N = 59.5%:40.5%. The whole conformational parameters (P, chi, gamma, delta, beta, epsilon, nu max) were analyzed as well as the energies of the OH...O intramolecular H-bonds on the basis of nu(OH) stretching vibrations. Convolution of calculated IR spectra of all of the T conformers appears consistent with its low-temperature matrix spectrum (Ivanov et al. Low Temp. Phys. 2003, 29, 809). The maximal discrepancy in frequencies between calculated and experimental spectra is less than 1%. A conclusion was made that for reliable reconstruction of the isolated nucleoside IR spectrum the quasi whole set of conformers should be taken into consideration. In essence, this result opens up a possibility to reconstruct IR spectra of isolated nucleosides at physiological temperatures with rather satisfactory probability.

9-(1,3-Anhydro-β-D-psicofuranosyl)adenine

In the structure of the title compound, C11H13N5O4, the glycosidic torsion angle, chi, is -107.1 (2) degrees [nucleic acid nomenclature used throughout the manuscript; IUPAC-IUB Joint Commision on Biochemical Nomenclature (1983). Eur. J. Biochem. 131, 9-15], indicating the anti conformation. The furanosyl ring adopts an N-type sugar pucker with the following pseudorotational parameters: PN = 50.5 (2) degrees and nu max = 34.9 (1) degrees. The conformation around the C5'-C6' bond is ap (gauche,trans; gt; -g), with a torsion angle gamma of 176.28 (19) degrees. The 1',2'-oxetane ring is not planar but folded along C3'...C1', with an angle of 9.6 (1) degrees.

Numerical study of confined slot jet impinging on porous metallic foam heat sink

This study numerically investigates the impinging cooling of porous metallic foam heat sink. The analyzed parameters ranges comprise ε = 0.93/10 PPI Aluminum foam, L/ W = 20, Pr = 0.7, H/ W = 2–8, and Re = 100–40,000. The simulation results exhibit that when the Re is low (such as Re = 100), the Nu max occurs at the stagnation point (i.e. X = 0). However, when the Reynolds number increases, the Nu max would move downwards, i.e. the narrowest part between the recirculation zone and the heating surface. Besides, the extent to which the inlet thermal boundary condition influences the prediction accuracy of the Nusselt number increases with a decreasing H/ W and forced convective effect. The application ranges of H/ W and Re that the effect of the inlet thermal boundary condition can be neglected are proposed. Lastly, comparing our results with those in other studies reveals that the heat transfer performance of the Aluminum foam heat sink is 2–3 times as large as that without it. The thermal resistance is also 30% less than that of the plate fin heat sink for the same volumetric flow rate and the 5.3 mm jet nozzle width. Therefore, the porous Aluminum foam heat sink enhances the heat transfer performance of impinging cooling.

Estimation of Intracellular Phosphate Content in Plant Cell Cultures Using an Extended Kalman Filter

Phosphate is an essential nutrient that is usually taken up by plant cells rapidly and stored intracellularly. Currently, there is no convenient means for on-line sensing of the intracellular phosphate content in cultured plant cells. In this study, a state estimator for this important parameter in batch plant cell cultures was developed using extended Kalman filter (EKF) methodology. A non-linear kinetic model was constructed to describe the dynamics of intracellular phosphate uptake and utilization. For intracellular phosphate estimation, this model was found to be most sensitive to three parameters: the maximum specific growth rate (mu(max)), the maximum phosphate uptake rate (nu(max)), and the yield coefficient on oxygen (y(o)). The EKF algorithm coupled with the kinetic model and on-line oxygen uptake rate (r(o)) measurement was used successfully to track the intracellular phosphate content under different initial phosphate concentrations. The state estimator could also accurately predict the biomass concentration. When mu(max), nu(max), and y(o) were included in the state vector, tracking of intracellular phosphate was only slightly affected. The estimation system was found very stable as long as the measurement errors of the initial states, the r(o) measurement error, and the system error were respectively within 30%, 30%, and 50%. With a r(o) measurement interval as long as 12 h, accurate tracking of the intracellular phosphate content could still be attained using a discrete EKF. Apparently, the slow r(o) dynamics in plant cell cultures allows the use of a long measurement interval. Considering the difficulties encountered in the on-line sensing of intracellular phosphate using the chemical or nuclear magnetic resonance (NMR) methods, the EKF method coupled with simple on-line oxygen uptake rate measurement provides a promising means for sensing this important culture parameter on-line.

Aspect ratio effect on natural convection in water near its density maximum temperature

Abstract The effect of the aspect ratio on natural convection in water subjected to density inversion has been investigated in this study. Numerical simulations of the two-dimensional, steady state, incompressible flow in a rectangular enclosure with a variety of aspect ratios, ranging from 0.125 to 100, have been accomplished using a finite element model. Computations cover Rayleigh numbers from 103 to 106. Results reveal that the aspect ratio, A, the Rayleigh number, Ra, and the density distribution parameter, R, are the key parameters to determine the heat transfer and fluid flow characteristics for density inversion fluids in an enclosure. A new correlation for predicting the maximum mean Nusselt number is proposed in the form of Nu max =a−b log (A) , with the constants a and b depending on density distribution number R. It is demonstrated that the aspect ratio has a strong impact on flow patterns and temperature distributions in rectangular enclosures. The stream function ratio Ψinv/|Ψreg| is introduced to describe quantitatively the interaction between inversional and regular convection. For R=0.33, the density inversion enhancement is observed in the regime near A=3.

An Independent Method to Determine the Height of the Mixed Layer

A method for the independent evaluation ofmixed-layer (ML) height, zi, has beenproposed. The ML height is determined bythe functional relationshipszi = 0.75 z/nv max or 0.53 z/nu max in which, z is a measuring height; nv max and nu max are normalized peak frequencies of lateral velocity component, v, and longitudinal velocity component, u, spectra at height z in the surface layer respectively. Using Doppler sodar data, the technique was shown to be feasible; it is easy to apply to micro-meteorological field experiments and works even for the ML top above the range of the sodar.

FTIR analysis of the interaction of azide with horse heart myoglobin variants.

The interaction of azide with variants of horse heart myoglobin (Mb) has been characterized by Fourier transform infrared (FTIR), electron paramagnetic resonance (EPR), and UV-VIS absorption spectroscopy and by molecular modeling calculations. Distal histidine variants (His64Thr, His64Ile, His64Lys) and charged surface variants (Val67Arg, Lys45Glu, Lys45Glu/Lys63Glu) were included in this study. All variants, with the exception of Val67Arg, have a lower azide affinity than the wild-type protein. Analysis of the temperature dependence of the FTIR spectra (277-313 K) revealed that the wild-type protein and all variants exhibit a high-spin/low-spin equilibrium. Introduction of positively charged amino acid residues shifts nu max for the low-spin form to higher energy while negatively charged residues shifted this maximum to lower energy. The low azide binding affinity exhibited by the His64Thr and His64Ile variants is accompanied by a shift of the nu max for the low-spin infrared band to lower energy and by a significant increase in the corresponding half-bandwidths. This observation indicates greater mobility of the bound azide ligand in these variants. The His64Lys variant exhibits two infrared bands attributable to low-spin forms that are assigned to two different conformations of the lysyl residue. In one conformation, the lysine is proposed to form a hydrogen bond with the bound azide similar to that proposed to occur between the distal histidine and bound azide, and in the other conformation no interaction occurs.

Identification of crotonaldehyde as a hepatic microsomal metabolite formed by alpha-hydroxylation of the carcinogen N-nitrosopyrrolidine.

Crotonaldehyde (2-butenal), which reacts with DNA and is mutagenic and carcinogenic, was identified as a hepatic microsomal metabolite of the hepatocarcinogen N-nitrosopyrrolidine. Incubation mixtures of N-nitrosopyrrolidine, cofactors, and hepatic microsomes from Aroclor pretreated or control F344 rats were derivatized with (2,4-dinitrophenyl)hydrazine reagent and the resulting mixtures analyzed by high-performance liquid chromatography. Crotonaldehyde (2,4-dinitrophenyl)hydrazone was identified by its retention time in two different systems and by its ultraviolet and mass spectrum. The ratio of 4-hydroxybutyraldehyde, which has previously been identified as a metabolite of NPYR, to crotonaldehyde was 1.5-2 over a range of substrate concentrations. The approximate values of Km and nu max for crotonaldehyde were 5.8 mM and 0.6 nmol/min/mg of protein and for 4-hydroxybutyraldehyde 14.1 mM and 1.7 nmol/min/mg of protein, for substrate concentrations between 1 and 8 mM, with microsomes from Aroclor pretreated rats. The ratio of 4-hydroxybutyraldehyde to crotonaldehyde was 1.9 upon esterase-catalyzed solvolysis of alpha-acetoxy-N-nitrosopyrrolidine, a stable precursor to the initial product of N-nitrosopyrrolidine alpha-hydroxylation. These results demonstrate that crotonaldehyde is formed upon metabolic alpha-hydroxylation of N-nitrosopyrrolidine and suggest that it may be involved in N-nitrosopyrrolidine-macromolecule interactions.

Heat transfer between fluidized beds of large particles and horizontal tube bundles at high pressures

The experimental heat transfer data for horizontal square inline tube bundles ( D T = 13.0 mm and pitch values are 19.5, 29.3 and 39.0 mm) immersed in fluidized beds of glass beads ( d ̄ p = 1.25 and 3.1 mm) and sands( d ̄ p = 0.794 and 1.225 mm) are measured as a function of fluidizing velocity and system pressure (1.1, 2.6, 4.1 and 8.1 MPa). The heat transfer coefficient values are found to increase with particle diameter, system pressure but are almost independent of tube pitch in the range investigated here. The h w values are compared with the predictions of five different theories available in the literature. The two correlations for Nu max are also considered and evaluated. Significant conclusions are drawn on the basis of reported h w data for large particles at high pressures.

Heat transfer between a gas-solid fluidized bed and a small immersed surface

In this paper are presented the results of an investigation of heat transfer from a small electrically-heated element to the bulk of a gas-fluidized bed. Mean and instantaneous values of the heat transfer coefficient were measured using various gases and solids at bed static pressures ranging from 0.03 to 1.48 MN/m 2. For all materials and conditions the heat transfer coefficient showed the same general dependence on gas flowrate. The heat transfer coefficient exhibited a maximum and the corresponding Nusselt number was related to the Galileo number and the Prandtl number of the gas by the equation Nu max = 0.30 Ga 0.20 Pr 0.40 1<Ga<220 From a trace of the element temperature against time, the corresponding instantaneous values of the heat transfer coefficient were calculated. At any gas flowrate the mean heat transfer coefficient could be expressed in terms of the coefficient at the incipient fluidization point and a term which was characteristic of the fluctuations.

Heat transfer to a sphere immersed in a shallow fluidized bed

By recording the transient response of spherical, calorimetric probes, of low Biot number, when plunged into shallow, heated, air-fluidized beds, heat transfer coefficients have been measured at bed temperatures of up to 1100°C. Bed heating was by means of premixed gas combustion, but all measurements were carried out in its absence. Particles of silica sand, zircon sand and silicon carbide, ranging in size from 200 μm to 800 μm, were investigated. A correlation, fitting some 400 data points to within ±10%, was obtained as Nu max = 0.365 θ 0.82 Ar 0.22 where θ = absolute bed temperature in o K/273 .