The stability of the coating was evaluated by conducting the boiling performance study for three consecutive runs. The sample S5 tested was prepared at 400 mg/l of concentration of AgNO3 and 5 min of immersion time. The performance of heat transfer got degraded from run 1 to run 3 in negligible magnitude, ascertaining the stability of the coating and its efficacy. Later, the surface morphology was investigated to look for alteration caused due to the boiling phenomena. The SEM images before and after pool boiling can be seen in Fig. 14 (b) and (c), which display negligible changes, and the surface properties remain intact.
Before embarking on a parametric study, the present study was compared with the literature to ensure the agreement and validation of the experimental result. In agreement, the experimental result of bare surface was found to be in compliance with the published works of AM Gheitaghy et al.[25], CM Patil et al.[6], S Mori et al.[37], Das et al.[3], Kim et al[38], AM Rishi et al.[5], and Vishal VN et al.[39], Akbari et al.[26], Esfahani et al.[28], as listed in Fig. 15.
4.4. Bubble visualization
The emanation of the bubbles from the surface is influenced by the surface morphology parameters such as porosity, nucleation sites, surface area, and rigidity of the coating. The rigidity of the coating is salient and has an immense effect on the transfer of heat from the substrate to the coating, since it happens through conduction. The nucleation sites are inseparable from the surface temperature of the coating, since the nucleation of the bubble is a function of temperature. The visualization of the bubble departure by using a high-speed camera is carried out to measure features such as bubble departure diameter, bubble departure frequency, and bubble density. For the comparative study, the bubble growth was observed with a high-speed camera (Phantom MIRO LAB 110) at 850 frames per second for bare (S1), microporous copper deposit (S2), and microporous copper covered by silver dendrites (S5). The bubble departure was captured sidewise to have a complete trajectory of bubbles from nucleation to escape to the water-atmosphere interface.
In the first stage, discrete bubbles are observed for microporous copper and sample S5, as illustrated in Fig. 16 (a) at lower heat flux. The intensity of bubble colonies was similar in both cases due to abundant micropores distributed over the surface. Since the nucleation sites are isolated as micropores, the bubble tends not to coalesce, it facilitates miniature bubbles rather than vapor slugs of larger size. On the copper disc of 25mm diameter, only 20mm diameter was coated by electrodeposition technique. The remaining area remains uncoated, aiding the handling of the sample. So, water also comes in contact with this portion to produce larger bubbles seen in Fig. 16 (c), which is undesirable. These larger bubbles attract a certain number of smaller bubbles emanating from the microcavities because of the affinity created by the pressure difference.
At the same situation, for the application of lower heat flux, the nucleation sites were insignificant over the bare copper surface conveyed by the image shown in Fig. 15 (c). The heat flux is insufficient to activate the nucleation sites, which is reflected in terms of higher onset of nucleation temperature and poor heat transfer coefficient, as seen in Fig. 12 (a) and (b).
When the heat flux was increased further, more nucleation sites were activated on microporous copper and sample S5, as shown in Fig. 17 (a) and (b). As a result, the bubble colonies were higher, and they absorbed an enormous amount of heat to yield a substantial heat transfer coefficient. However, the HTC and wall superheat of sample S5 is invariably greater than microporous copper, which is attributed to the presence of silver dendrites. The silver dendrites contribute to additional nucleation sites so that sample S5 has the upper hand than S2. Further increasing the heat flux to higher values leads to excessive growth of bubbles in a larger size. Also, the interruption of the relatively larger bubbles from the uncoated portion, advertently left for handling, tends to interact with the bubbles formed from the coatings. This unfortunate event makes the smaller bubble generated at moderate heat flux unidentifiable. Approval for this statement, in Fig. 17 (a), the marked portion corresponds to smaller bubbles generated, and the majority of them are obstructed by the larger bubbles from the uncoated portion. At higher heat flux, which can deliver higher local heat flux, activation of nucleation sites is enormous, which leads to the transformation from an isolated bubble regime to the regime of slugs. It is because the vacant vicinity of each individual bubble shrinks with an increase in the local heat flux as a greater number of nucleation sites are activated adjacently so that the isolated bubble regime sustains only a shorter spectrum of heat flux.
The bubble visualization was quantitatively investigated by measuring the parameters of bubble departure diameter, bubble departure frequency, the bubble nucleation density. The nucleate boiling regime was classified into a regime with isolated bubbles and a regime of slugs and columns based on the bubble nature[3, 40]. Only one heat flux was evaluated in the isolated bubble regime, where the size, frequency, and density were able to calculate. The reported bubbles diameter and bubble duration were the average of twenty different measurements, and the diameter of the copper testing specimen was used as the reference dimension. For sample S5, the bubble departure frequency, which is the inverse of the time interval between two successive bubble departures, was found to be 158 sec− 1. This was calculated by counting the number of frames between two successive bubble departures since the time for each single frame is known. Since the bubble departure per second is 158 Sec− 1, the enhancement is substantial when compared with a bare surface. In rational terms, the bubble departure frequency of sample S2 was expected to be lower than sample S5 due to the excessive nucleation sites pertaining to S5. As expected, the bubble departure frequency, f, for sample S2 was found to be 76 sec− 1, which was half the value of the f calculated for sample S5. Similar to the bubble departure frequency, the bubble departure diameter corresponding to sample S5 was equally intriguing. Upon sampling a hundred bubbles at ten different frames, the average bubble departure diameter was 0.71mm for the sample S5. In the case of sample S2, microporous copper, the bubble diameter was 0.88mm which is higher than the former case, and the hike is notable in the range considered here. Also, it is indirectly understood that the larger bubble diameter takes a larger time for growth and departure, as is reflected in the bubble departure frequency.
At higher heat flux, the bubble regime shifted to slug and column due to the coalescence of the bubble nucleus triggered by enormous nucleation sites. The visualization revealed that the vapor clouds are fragmented in the case of sample S5, whereas it is a single massive could in the case of bare copper, as seen in Fig. 17 (c) and (d), respectively. Likewise, the fragmented bubble leads to lower bubble size and lower bubble departure frequency, and vice versa for bare. Due to delayed bubble coalescence in sample S5, the vapor cloud was in fragments of smaller slugs, whereas intense coalescence in the bare led to larger vapor clouds. From the calculation, the bubble departure frequency for sample S5 was 18.86s− 1 and 13.33s− 1 for the bare copper surface. The higher departure frequency, even at higher heat flux for sample S5, is the corollary of a higher heat transfer coefficient due to the enhancement achieved through surface modification of the bare surface with microporous copper decorated with silver dendrites.