Saturated nucleate pool boiling experiments are conducted in de-ionized, distilled water and in aqueous polymeric solutions over a horizontal, cylindrical heater. Boiling characteristic (qw'' vs. ΔTsat) of water, at atmospheric pressure, is first established by conducting experiments over an extended period of time and confirming repeatability of the experimental results. Aqueous solutions of three grades of HEC (Hydroxyethyl Cellulose) polymer viz. 250-HR (1000 kg/mol), 250-MR (750 kg/mol) and QP-300 (600 kg/mol), are then used at varied concentrations in a series of nucleate pool boiling experiments, so as to study the effect of pseudo-plasticity on boiling heat transfer.
Polymers, when dissolved in water, change the rheological and interfacial properties of the solution and affect the ebullient boiling behavior. This viscous non-Newtonian, shear-thinning solution also displays interfacial tension relaxation, which tends to be both concentration dependent and temporal. A corresponding increase in surface wettability (smaller contact angle) is also observed. The boiling behavior in aqueous polymer solutions is found to be significantly influenced by changes in the wetting, vapor-liquid interfacial tension, and shear-thinning viscosity of the polymeric solutions. Both the concentration of the polymer and its degree of polymerization (which is reflected in its molecular weight and rheology) have an effect on the heat transfer and associated bubble dynamics.
The effect of concentration (1.0 x 10-9 ≤ C ≤ 4.0 x 10-9 mol/cc) are seen in the nucleate boiling characteristics of aqueous solutions of HEC QP-300 (M ~ 600 kg/mol). The measured pool boiling heat transfer from the electrically heated horizontal cylinder in C = 1.0 x 10-9 mol/cc (~ critical polymer concentration, C*, for HEC QP-300) aqueous solution is found to be enhanced by ~ 20 % over the entire heat flux range (4.0 < qw'' < 200 kW/m2). In higher concentration solutions, however, heat transfer deteriorates at low hear fluxes (or in the incipience and partial boiling regime). At high heat fluxes or in the fully-developed nucleate boiling regime, on the other hand, heat transfer enhancement (~ 45 % maximum) is obtained. This anomalous boiling behavior in the two regimes is characterized by respectively different ebullience signature (as depicted by photographic imaging). Also, it is shown to be scaled with changes in the liquid-solid interface wetting, vapor-liquid interfacial tension, and shear-thinning viscosity of the polymeric solutions.
The effects of liquid pseudo-plasticity and dynamic interfacial tension are seen in the Saturated nucleate pool boiling of aqueous solutions of three grades of the HEC polymer, namely, 250-HR (1000 kg/mol), 250-MR (750 kg/mol), and QP-300 (600 kg/mol). The experiments are conducted at constant molar concentration of C = 2.5 x 10-9 and 4.0 x 10-9 mol/cc, for all the three polymer grades. With C = 2.5 x 10-9 mol/cc solution, boiling heat transfer coefficient is found to decrease even below that of water in the partial boiling regime (4.0 ≤ qw'' ≤ 20 kW/m2). The reduction increases with viscosity. However, at higher heat flux, the shear-thinning of the polymeric solutions diminishes the viscous effects giving enhancement up to 32 %. The lower molecular mass HEC QP-300 has a better performance than the large-chain 250-MR and 250-HR because of its lower dynamic surface tension in the high-frequency ebullience regime of fully-developed boiling.
|Commitee:||Jog, Milind, Kao, Yuen, Manglik, Raj|
|School:||University of Cincinnati|
|School Location:||United States -- Ohio|
|Source:||MAI 57/06M(E), Masters Abstracts International|
|Keywords:||Boiling, Boiling curve, Hydroxyethyl cellulose, Polymer, Polymer boiling, Pool boiling|
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