Refrigerant boiling at low heat flux in vertical tubes with heat transfer enhancing fittings

• The boiling process in vertical tubes with heat transfer enhancing inserts.
• A comparison of heat transfer coefficients and pressure drops.
• The effectiveness of heat transfer enhancement by three types of inserts.
• The relationships to calculate the heat transfer coefficients and flow resistance.

The paper presents the results of thermal and flow analyses of the boiling process in vertical tubes with heat transfer enhancing inserts. Tests were performed for three different geometrical shapes of the inserts: a spiral tape with a core rod, a rib insert and a spring insert. Experimental measurements were performed for R507, R410A and R407C refrigerants at low heat flux. The obtained results indicate the increase ratio of heat transfer and flow resistance coefficients in tubes with inserts, as opposed to a plain one. Finally, the paper presents the dimensionless relationships which enable the calculation of heat transfer coefficients and pressure drops during boiling in vertical tubes with the studied heat transfer enhancing inserts.