Combined Visualization and Heat Transfer Measurements for Steam Flow Condensation in Hydrophilic and Hydrophobic Mini-Gaps

Abstract

Condensation enhancement was investigated for flow condensation in mini-channels. Simultaneous flow visualization and heat transfer experiments were conducted in 0.952-mm diameter mini-gaps. An open loop steam apparatus was constructed for a mass flux range of 50-100 kg/m 2 s and steam quality range of 0.2-0.8, and validated with single-phase experiments. Filmwise condensation was observed in the hydrophilic minigap; pressure drop and heat transfer coefficients were compared to the (Kim and Mudawar, 2013, "Universal Approach to Predicting Heat Transfer Coefficient for Condensing Mini/Micro-Channel Flow," Int. J. Heat Mass Transfer, 56(1-2), pp. 238-250) correlation and prediction was very good; the mean absolute error (MAE) was 20.2%. Dropwise condensation was observed in the hydrophobic mini-gap, and periodic cycles of droplet nucleation, coalescence, and departure were found at all mass fluxes. Snapshots of six typical sweeping cycles were presented, including integrated flow visualization quantitative and qualitative results combined with heat transfer coefficients. With a fixed average steam quality ( x ¼ 0.42), increasing mass flux from 50 to 75 to 100 kg/m 2 s consequently reduced average sweeping periods from 28 to 23 to 17 ms and reduced droplet departure diameters from 13.7 to 12.9 to 10.3 lm, respectively. For these cases, condensation heat transfer coefficients increased from 154,700 to 176,500 to 194,800 W/m 2 K at mass fluxes of 50, 75, and 100 kg/m 2 s, respectively. Increased mass fluxes and steam quality reduced sweeping periods and droplet departure diameters, thereby reducing liquid thickness and increasing heat transfer coefficients.

Copper-Based Superhydrophobic Nanostructures for Heat Transfer in Flow Condensation

“…While both plain hydrophobic and biphilic surfaces provided enhancements in the thermal performance compared to that of the bare hydrophilic surface, the best thermal performance was observed for the plain hydrophobic surface. Chen and Derby extended Derby’s previous work, and their heat transfer analysis and visualization study on a minichannel revealed enhancements in the thermal performance of hydrophobic surfaces, as a result of the increased droplet mobility. Another study by Chen et al on flow condensation heat transfer inside a minichannel with a hydrophobic surface in the presence of non-condensable gases (NCG) revealed that NCG reduced the thermal performance of the system by 24–55%, which was due to the accumulation of NCG pockets on the condensing surface.…”

“…Therefore, reducing the channel cross sectional area helps increase vapor mass fluxes and obtain higher vapor shear rates. To investigate the potential of hydrophobic surfaces for enhancing the steam flow condensation heat transfer performance in minichannels and microchannels, some studies could be found in the literature. − In the study of Fang et al, flow visualization and heat transfer analysis of flow condensation of pure steam were performed in a rectangular microchannel, and a strong dependency of flow patterns on wall wetting properties were reported . Derby et al investigated steam flow condensation in a minichannel with various biphilic patterns for steam mass fluxes in the range of 50 to 200 kg/m 2 s and a wide range of steam qualities.…”

Mini-channel flow condensation enhancement through hydrophobicity in the presence of noncondensable gas

“…In order to study the impacts of noncondensables on steam flow condensation, nitrogen was introduced to condensing steam in an open-loop experimental apparatus [48], shown in Figure 1.…”

“…Reduced channel size (i.e., mini-and microchannels [39][40][41][42][43] and hydrophobicity [44][45][46][47][48] are two heat transfer enhancement approaches studied in this paper. In mini-and micro-channels, surface tension becomes significant and changes the flow regime to improve heat transfer, and increased surface hydrophobicity promotes dropwise condensation and facilitates liquid removal [48][49][50]. To understand the influence of steam flow conditions, noncondensable gas fractions, and wettability on condensation heat transfer, this work studies steam condensation heat transfer in 1.82-mm, hydrophilic and hydrophobic rectangular mini-channels at steam mass fluxes of 35-75 kg/m 2 s, steam qualities of 0.3-0.9, and NCG mass fractions from 0-30%.…”

Visualizing and disrupting liquid films for filmwise flow condensation in horizontal minichannels

“…For condensation processes, the condensed liquid (i.e., condensate) acts to insulate the cooling surface from the working fluid (Sun and Wang, 2016;Niu et al, 2017;Alizadeh-Birjandi et al, 2019). While condensation can be dropwise or filmwise, filmwise condensation is prevalent in internal flows as it does not require any special coatings or conditions to occur (Ma et al, 2014;Chen and Derby, 2016;Alizadeh-Birjandi et al, 2019). Since the film in filmwise condensation prevents direct heat transfer between the cooling surface and the vapor, heat transfer coefficients tend to be an order of magnitude lower compared to dropwise condensation (Rose, 2002;Orejon et al, 2017;El Fil et al, 2020).…”