Critical Heat Flux and Minimum Heat Flux of Film Boiling of Binary Mixtures Flowing Upwards in a Vertical Tube

The effects of heated length, local quality or subcooling and mole fraction at constant pressure (1.1 MPa) and mass flux (ṁ = 1000 kg/(m²s)) on the critical heat flux (CHF) and the minimum heat flux of film boiling (MHF) have been investigated for the refrigerant mixture CBrF₃(R13B1)/C₂Cl₂F₄ (R114). The study is restricted to the DNB type of the boiling crisis. It was found that CHF has a maximum in a CHF versus mole fraction plot. The higher the outlet subcooling the more pronounced is the maximum. Only in long heated test sections and at an outlet quality of ẋ = 0.2 no maximum was observed. There is also a heated length effect. For the pure components : (l/d)_min≈ 10, above which the CHF does not change for fixed outlet subcooling or quality. The corresponding limit for mixtures was found to be significantly larger: (l/d)_min≈ 30. All above mentioned effects are of minor importance for MHF.