Abstract
A range of experiments was conducted to measure the heat transfer characteristics of titanium oxide/deionized water nanofluid (NF) inside a steel-made Pyrex annular system. A set of experiments was designed and performed at inlet temperature (IT) of the NF (333 K-363 K), the applied heat flux (AHF) (4.98 kW/m2 to 112 kW/m2), 1988 < Re < 13,588 and dispersion concentration of wt.%=0.05 to wt.%=0.15) on the average heat transfer coefficient (HTC) and boiling section’s average pressure drop (PD). It was demonstrated that the increase in the volume flow and the AHF can increase the HTC while increasing the weight concentration of the NF, initially increased the HTC such that the maximum enhancement in the HTC was 35.7% at wt.%=0.15 and Re=13500, however, over the time, the HTC of the NF decreased. The reduction in HTC was attributed to the formation of continual sedimentation on the boiling surface after 1000 minutes of the operation. The IT of the NF slightly increased the HTC, which was due to the enhancement in the thermal and physical properties such as thermal conductivity. The maximum enhancement in HTC due to increase of the IT from 333 K to 363 K was 4.2% at wt.%=0.15 and Re=13500. The bubble formation was also found to be a strong function of the applied HF such that with increasing the HF, the rate of the bubble formation increased, which was also the reason behind the augmentation in the HTC at larger AHFs. Also, the PD was augmented due to the increase in the velocity and flow and also weight concentration of NF. The highest value measured for PD was 9 kPa recorded at a weight fraction of 0.15 and Re=13500, which was 28% larger than that of measured for the base fluid. It was also found that a continual fouling layer of nanoparticles (NPs) was formed on the boiling surface, which induced a thermal resistance against the boiling heat transfer. The fouling formation reduced the HTC of the NF such that the maximum reduction in the HTC was 21.6% after 1000 minutes of the operation of the heater.
Highlights
Boiling heat transfer and bubble formation are wonderful phenomena with lots of complex mechanisms and sub-phenomena, which are most of the time unknown and need further research [1, 2]
Sarafraz et al [35, 37, 47] conducted a set of experiments on the flow boiling heat transfer of a group of metal oxides including copper oxide, alumina, titanium oxide, silica and noticed that the heat transfer coefficient (HTC) was a transient parameter, which can be regulated by controlling the thermal fouling resistance
The first layer of liquid is evaporated and form a very small bubble which normally cannot be seen without visualization equipment. This is the point of separation for both heat transfer mechanisms, which is called the Onset of Nucleate Boiling
Summary
Boiling heat transfer and bubble formation are wonderful phenomena with lots of complex mechanisms and sub-phenomena, which are most of the time unknown and need further research [1, 2]. In contrast to the above studies, for the systems working in the two-phase flow, the presence of the nanoparticles can apply a thermal resistance which suppresses the heat transfer mechanisms on the surface Apart from this issue, there are extensive studies in which it has been shown that HTC can be augmented due to the NPs [7, 9, 3339]. Sarafraz et al [35, 37, 47] conducted a set of experiments on the flow boiling heat transfer of a group of metal oxides including copper oxide, alumina, titanium oxide, silica and noticed that the HTC was a transient parameter, which can be regulated by controlling the thermal fouling resistance He noticed that the presence of the NPs within the liquid phase reduced the HTC value. The generation of the bubbles inside the base fluid was investigated as well
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