The circuit boards of an electronic equipment were simulated with a vertical channel which had thermal blocks protruded from one of the channel walls. A rough front plate was made of a circuit board attached with short wires to simulate the back side of a printed circuit board. Natural convection experiments were carried out to study the effects of the channel space and rough front plate and to find the suitable characteristic length used in non-dimensional numbers. Heat transfer coefficients were converged to the asymptotic value after the fourth row. The effect of rough front plate was negligible. There were negligible effects of the channel space on the first and second heaters. Heat transfer coefficients after the third row decreased as the channel space decreased. Heat transfer coefficients were almost constant for the channel space larger than 20 mm. A characteristic length was suggested to non-dimensionalize Nu and Ra numbers in a vertical channel with protruded heaters. A correlation was obtained using the new characteristic length.