Laminar forced convection in air (
Pr=0.7) in two-dimensional wavy-plate-fin channels with sinusoidal wall corrugations is numerically simulated. Constant property, periodically developed flow in ...uniform wall temperature plate channels is considered. Velocity and temperature fields, isothermal Fanning friction factor, and Colburn factor are presented for different flow rates (10⩽
Re⩽1000), wall-corrugation severity (0.125⩽
γ⩽0.5), and fin spacing (0.1⩽
ε⩽3.0). Lateral vortices or re-circulation cells develop in the wavy-wall troughs as the axial flow gets separated downstream of the wall-corrugation peaks and re-attaches upstream of the subsequent wall peak, and their strength and coverage is a function of
Re,
γ, and
ε. As the plate separation
ε decreases, however, viscous forces dominate and dampen the swirl; with very large
ε, the impact of wall waviness diminishes and the core fluid flows largely undisturbed. The surface waviness-induced periodic disruption and thinning of boundary layers along with lateral swirl mixing produce high local heat fluxes near the wall peak regions. The overall heat transfer coefficient increases many fold compared to that in flat-plate channels with relatively small increases in the concomitant pressure drop penalty. The optimum (
j/
f) enhancement is obtained in the swirl flow regime (
Re>100) with
γ>0 and 1.0⩽
ε⩽1.2.
The effect of surfactant molecular mass transport on the normal impact and spreading of a droplet of its aqueous solution on dry horizontal substrates is investigated experimentally for a range of ...Weber numbers (20−100). The postimpact dynamics of film spreading and its recoil behavior are captured using high-speed real-time digital imaging. Hydrophilic (glass) and hydrophobic (Teflon) substrates were used with water and aqueous solutions of three different surfactants of varying diffusion rates and ionic characteristics: SDS (anionic), CTAB (cationic), and Triton X-100 (nonionic). Their solutions facilitate larger spread and weaker surface oscillations compared to a pure water drop colliding at the same Weber number. On a hydrophobic surface, the drop rebound and column fracture are inhibited by the presence of the surface-active agent. Besides reagent bulk properties, dynamic surface tension, surface wettability, and droplet Weber number govern the transient impact−spreading−recoil phenomena. The role of dynamic surface tension is evident in comparisons of impact dynamics of droplets of different surfactant solutions with identical equilibrium surface tension and same Weber number. It was observed that higher diffusion and interfacial adsorption rate (low molecular weight) surfactants promote higher drop spreading factors and weaker oscillations compared to low diffusion/adsorption rate (high molecular weight) surfactants.
Steady forced convection in periodically developed low Reynolds number (10
⩽
Re
⩽
1000) air (
Pr
=
0.7) flows in three-dimensional wavy-plate-fin cores is considered. Constant property computational ...solutions are obtained using finite-volume techniques for a non-orthogonal non-staggered grid. Results highlight the effects of wavy-fin density on the velocity and temperature fields, isothermal Fanning friction factor
f, and Colburn factor
j. The fin waviness is seen to induce the steady and spatially periodic growth and disruption of symmetric pairs of counter-rotating helical vortices in the wall-trough regions of the flow cross-section. The thermal boundary layers on the fin surface are thereby periodically interrupted, resulting in high local heat transfer near the recirculation zones. Increasing fin density, however, tends to dampen the recirculation and confine it. The extent of swirl increases with flow rate, when multiple pairs of helical vortices are formed. This significantly enhances the overall heat transfer coefficient as well as the pressure drop penalty, when compared to that in a straight channel of the same cross-section. The relative surface area compactness as measured by the (
j/
f) performance or the area goodness factor nevertheless increases with fin density.
An experimental investigation, coupled with theoretical modeling of CaCO
3
fouling in plate-and-frame type heat exchangers (PHEs) have been conducted. Four different plates, made of SS-304, are used ...in two different surface patterns (chevron and zig-zag) of varying corrugation severity (waviness depth and pitch) and area enhancement. They were further characterized in clean, non-fouled convection by their measured heat transfer coefficients and friction factors in the Reynolds number range of 600–6000. The flow-fouling experiments delineate the effects of temperature and plate-surface geometry on growth rates and stabilization of fouling resistance, along with the anti-fouling behavior of plates coated with a hydrophobic PTFE (Teflon) film. Moreover, the microscopic structure of fouling deposits is mapped in a scanning-electron microscope. Corrugated plates with the largest height-to-pitch ratio and hydraulic diameter are found to have the lowest fouling growth rate and resistance; Teflon-film coating of plate surface is also found to mitigate fouling relative to the performance of bare stainless steel plates. Finally, a semi-empirical fouling model, based on the Prandtl–Taylor analogy, has been devised to describe the experimental data and provide a predictive tool.
Thermal energy storage, in the form of chilled water or ice-based phase-change systems, has been used in commercial buildings for over 30 years. These systems take advantage of off-peak electricity ...rates to cool water or ice at night, which provides cooling power during the day when the electricity prices are at their peak. Although these systems work well for commercial buildings, they are not feasible for residential applications, mainly due to the fact that a residential system supplies air at 55degreesF (12.8degreesC)--a temperature that is too high to take advantage of the latent heat of water which freezes and melts at 32 degreesF (0degreesC); therefore, alternative phase-change materials are needed. The direct implementation of phase-change materials becomes problematic due to the very nature of these materials cycling between solid and liquid states during normal operating temperatures. Therefore, encapsulation of the phase-change material, typically in a polymer, is necessary to maintain structural shape stability during the melting and solidification of the material. Polymers used for encapsulation help maintain the shape of the material as well as retain the phase-change material within the polymer during the phase change. Coincidentally, polymer heat exchangers and additive manufacturing are also of rising interest. Polymer heat exchangers boast a number of benefits including weight reduction, natural corrosion resistance, and anti-fouling when exposed to typical process fluids. The goal of the study is to explore the implementation of a novel, functional encapsulated phase-change material as the primary structure in a polymer heat exchanger, which can be additively manufactured, for residential thermal energy storage. The work presented includes an exploratory thermodynamic analysis on the effectiveness of the novel polymer phase-change material thermal energy storage heat exchanger on reducing the load on a residential HVAC unit.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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